This guide is meant as a resource both for people who are completely new to clan battles and people who want to get a little bit better at Clan Battle. If you're already in a high level clan you probably won't get anything out of this article, but hopefully we've got some tips for you if you're looking to squeeze out as much damage as you can.
The absolute basics:
Clan Battles are competitive events where clans battle bosses in an attempt to deal the most damage to them. Damage dealt is then converted into points and is used to tabulate the final score for a clan at the end of the event.
There are five bosses that must be fought one at a time and once all five are defeated, the bosses respawn and you fight them again from the beginning. After completing your first lap through the bosses, all future laps will be stronger "Stage 2" versions with various buffed stats. Over the coming months and years, harder and harder stages will be added to increase the challenge.
Each player in a clan may earn up to three attempts to fight bosses per day. The attempts are earned by using stamina in normal and hard stages. You earn one CP per stamina used, and every 300 CP earned lets you fight a boss one time. For the maximum of three runs per day, you would have to spend 900 stamina to earn 900 CP. Note that this is more than you can earn in a single day (with the current set of Guildhouse items as of writing), so you'll have to spend some jewels on refreshing your stamina to get your last hit.
For each attempt, you are given 90 seconds to do as much damage to the monster as possible. In addition, you may only use each character in your roster one time per day over all your attempts. This is where borrowing a fellow clan member's unit as support will be useful, since you are able to borrow each character from a clan member once per day by going to the "Support" tab in the team selection screen. For example, you can have Makoto in all three of your daily attempts by using your own Makoto and borrowing her two times from different members (or by borrowing her three times from three different members if you don't have her yourself).
Any additional time left over from when you finish off a monster is then carried over with some bonus time that you can use to fight another boss with the same team. There are further edge cases that can occur when multiple people are fighting a boss close to death and when you disconnect or your app crashes, and the details of that can be found in the in-game help menu (it's very thorough).
In conclusion, the event is a competition event when you try to do the most damage and gain the most points as a clan. The clans are then ranked against one another and rewards are given out based on your clan's ranking (the specific rewards can be found under "Ranking Rewards" on the clan battle ranking page or our previous clan battles page).
Just about everything listed here is aimed at people in clans that don't do any real coordination who want to level up their Clan Battle game. The tips below take varying amounts of time/effort investment and have varying levels of ROI, but the most important thing you can do is...
Test everything. The Trial Run feature exists for a reason. Use it. Every boss in CB has different mechanics and different stats. Just because a team works against one boss it doesn't mean it'll work against another boss. Units like Jun, Yukari and Kokkoro (Summer) target the unit with the lowest HP percentage with their heals, so running the same team vs different bosses, or running the same team after changing your equipment/rank could lead to wildly different results. Even if you're not fighting a different boss, the 5th boss will enrage at 50% HP and change its skill rotation (usually making the fight more difficult). If someone tells you a team comp, unless you know you've got the same units at the same stars at the same ranks with the same equipment and refinement with the same bond stories watched at the same level with the same skills leveled against the same boss, there's always a chance the team won't work for you. It takes about one minute to run a team full auto at 2x speed and that's usually enough to tell if your team completely falls apart. Don't let one minute of testing be the reason you do a stinker hit instead of a halfway decent one. If your run is falling apart because of an unlucky crit or miss, you can force quit the app and try again once per day.
Build the right units. Clan battle meta units typically do large damage to a single unit, boost TP, apply defense-down debuffs, increase sustain, interrupt boss mechanics, or some combination of the above. Figuring out which units actually work well and their ideal equipment setup is challenging, so a great starting point is Chocolate's spreadsheet which has unit importance and recommended gear. Another great resource for gear recommendations is Nyara's spreadsheet. Note for the new players: a rank like "9-6" means rank 9 with all 6 pieces of equipment equipped. Don't hit the Rank Up button, as that'll take you up to rank 10 with 0 pieces of equipment. Prioritizing units that work well for CB is far more important than keeping them at "ideal" lower ranks, but if you have any intention of joining competitive clans, it's a good idea to not rank units past these recommendations.
Stack defense-down debuffs. If you've been checking any other resources about Clan Battle, you know that Makoto is hailed as one of the most critical units for this mode. That's because her skill 2, Heroic Howl, applies a physical defense-down debuff, and her UB does the same in addition to doing decent damage. Each defense-down applied to the boss is not only additional damage for Makoto, it's additional damage for every physical attacker on the team. Typically the sweet spot for number of defense-down units on a team is 2, since too many gives up too much DPS and could ruin your later teams, but there are some cases where running 1 or 3 might make sense. Physical defense-down units include Makoto, Mitsuki, Kokkoro (summer), Jun, Shinobu, (and if you're really desperate) Karyl and Yuki. Magic defense-down units include Karyl (summer), Akari, and Karyl. If you're not sure which subset of these units to run with a given team, read on to the next point which is...
Synergize your teams. Specifically, try to run entirely physical or entirely magical teams. Say you put Makoto and Mitsuki, two great physical defense-down units, on a team together. If you also put Kyoka on that team, she may be a great damage dealer on her own, but as a magic unit she's not going to benefit from the physical defense-down that the other units are providing. For certain support roles (e.g. tank or healer) it may make sense to mix magic and physical, but your primary DPS units should match your defense-down units. Other things to pay attention to are units like Saren, who will charge the TP of the unit standing closest to her. You generally want to make sure that Makoto is the one who receives the TP charge, since more Makoto UBs means more defense-down, and more defense-down means more damage. The TP charge is mostly wasted on a unit like Arisa, who is already constantly UBing. With the current set of available units, most players should either run three physical teams or two physical teams and one magic team every day. Nearly every physical team should run Makoto and nearly every magic team should run Karyl (Summer), so if you're serious about Clan Battle you should have (or be farming for) Makoto, have S.Karyl or be in a clan where people have those units available as a support.
Don't be afraid to go tankless. Typical Princess Connect teams in story quests and arena all run tanks, but running those same teams against Clan Battle bosses could be costing you a lot of damage. Kaori, for example, stands very far forward and doesn't have the highest effective HP, but she has a massive physical attack stat, has HP drain, and because she's constantly getting hit, she UBs all the time. If you run her in front of your team against a boss that sometimes targets the back line, there's a good chance she survives. If Kaori isn't surviving, you can run Kokkoro on the same team and when she uses her Tri-Slash, manually UBing will cause Kokkoro to get stuck in front of your team, effectively turning her into a tank. Another common tactic is to run Makoto in front with Kokkoro (summer) on the same team. Kokkoro's defense down will synergize well with Makoto's, and her UB might come quick enough to keep Makoto alive. For magic teams, Illya often UBs quickly enough to heal herself before dying, but if she dies too quickly, running a Misato can massively increase Illya's survivability. How will you know if a team can survive without a tank? See tip #1.
Plan your hits out in advance. What this exactly means will vary from clan to clan, but the basic idea remains the same. You'll likely earn more points if you can hit multiple different bosses. This isn't always true (e.g. boss 5 is often vulnerable to multiple teams and has a very high score multiplier), but in general you'll be able to get the most out of your strong units by putting them up against different bosses. One boss might attack your back line, while another targets your units with the highest attack. Shiori is in danger in both of these situations, but she's also a really strong single target physical attacker, so if you can use her to hit a third boss that doesn't kill her, you might be able to do more damage than if you had hit one of the others. Obviously this won't apply to every clan and every player, as many of us are busy and don't have more than a couple hours a day to hit bosses, but a little planning goes a long way. Remember, your goal is to maximize the amount of points your clan earns, and if you're in a clan that doesn't coordinate hits, that generally means your goal is to maximize your personal points earned. You want the sum of your three hits to be as high as possible, so sometimes running three medium damage teams might make more sense than running a single high damage team. So how do you know what teams are good?
Look for resources from other servers. There's a wealth of information from all the other Princess Connect regions that people are compiling and translating. We here at Fansubbing aren't in the business of compiling specific team comps, but knowing where to look for public resources is a great place to start. Princess Connect YouTubers like LacieylG and Bork often make videos for specific upcoming clan battles, and they sometimes have links to spreadsheets or bilibili playlists with team comps from the CN, JP, and KR servers. Another good place to look is in the /r/Priconne Discord server. They compile a lot of useful information and put it in their #en-gameplay-guides channel and the pinned messages in #en-cb-chat. It's worth noting that the CN Princess Connect server ran a wildly different schedule from JP and Global, so going forward, the resources from that region won't be as useful. So you've found some team comps, now the next step is...
Learn how to use timelines. Most things I've written about so far have been about macro optimizations (mostly team composition), while timelines are a more micro optimization (given a team, how can I maximize the damage output?). Nearly all the resources you find will be from moderately high level players who are using manual mode to maximize the amount of damage they can do. Apart from random misses and random crits, Princess Connect is a fairly consistent game. Damage numbers are fixed based on the attacker's attack stat, the defender's defense stat, and the skill's base power. Because damage numbers are fixed, so are TP gain numbers, and because skill rotations are fixed, you can consistently tell when TP gauges will fill up and UBs become available. This consistency allows you to decide precisely when to use UBs to maximize your total damage. A list of UBs and when to use them is known as a timeline and it can be a very powerful tool for squeezing as much damage out of a team as you can. If you're in a clan that provides timelines the following probably doesn't apply to you, but say all you have is a bilibili video of a strong CB run that you want to try for yourself. In that case, it's note taking time. Jot down the times that each character UBs, as well as the various cues to look for. Does the player mash the UB button or do they wait for a skill to finish applying? Is the team running Yukari or other unit that targets skills based on current HP or TP? If the timeline isn't working for you, it could be that a TP charge or a heal is going to the wrong unit either because of a mistake in your execution, a random boss miss or crit, or a mismatched character rank. If it's the latter, you're probably going to have to find a new timeline to try. Given all that, actually creating your own timelines is outside of the scope of this article, but if that interests you, you should find a clan that values this level of optimization and will help you theorycraft.