Hi, I'm Krinel from guild Abaddon. I'm writing this guide to help guild mates from Avalon defeat Biffron and if others want to follow our example, they're more than welcome :). After acquiring Drake, I've been able to pump out 1 million damage to Biffron, the 6th boss of Chaos Abyss.
DISCLAIMER: I have not paid any money to acquire event heroes or items. This guide is not intended for players who spend real money to get boosted event heroes. This guide is solely for non-pay players. You don't need a guide to win when you have all event heroes on rank 6.
Bosses 1 to 5 are rather straightforward: you need to deal as much punishment as you can in a short time frame since your only worry with damage received comes from the exponentially-growing damage the boss deals over successive rounds.
Boss 6 and onwards are radically different. You now have a concern of survival from the mere damage dealt by the boss and its minions right off the start in round 2 (and perhaps even 1). If you don't mitigate damage dealt by the boss, you will not be able to make it past round 4. The only way to mitigate damage is through attack debuffs.
My team formation: Drake (double stun+def debuff), Sarah (def debuff), Sigrune (atk debuff), Erin (atk debuff), Ariesse (atk debuff), Jarkan (fury thief), Main axe (healer), Dinari (crit boost), Blood Pearl (dmg, event hero)
Your team formation: My team composition is justified by the rank of my different heroes, the equipment I have and my hero wars preferences. It'd be better for me to use the broken arrow spy in my formation against bosses, but I prefer Sarah in hero wars. I don't want to switch gear back and forth between the two heroes so I just use Sarah, even though she's not the optimal hero for damage dealing for me right now. It might turn out that you have 2 strong bows but no strong cannon - in that case, taking Sigrune + Erin as attack debuffers instead of Sigrune + Vanessa would make sense.
1 double-turn stunner: Currently, both Drake and Shyre stun for two turns. Drake, however, also debuffs defense by 40%, which is a huge advantage. I also believe Drake's stun rate is close to 100%, if not 100%
2 Defense debuffers: Hopefully you have Drake who can act as both a stunner and debuffer. One debuffer is not enough: you'll need at least Sarah/broken arrow spy who both debuff with 100% probability. Three defense debuffers might be an overkill (by the way, don't underestimate the 1-star broken arrow spy: with 4 stars, he is as deadly as any other season 1 hero with 6 stars!). Debuffers that do not have 100% chance of success such as Melissa and Vera should be totally avoided.
3 Attack debuffers: Three attack debuffers are necessary for consistent results. If you have 2 attack debuffers, you will most likely die by round 4 and be unable to apply the 2nd 2-turn stun, or will do so with only a few heroes available. Sigrune, Vanessa, Kruel, Mordred, Erin (300 fury though), and Ariesse (also 300 fury, but she's currently an event hero so she packs quite a punch!) are good choices here.
1 Fury Thief: Any fury stealer who targets a single ennemy will allow you to gain a much needed 300 fury (no Harold!). Jarkan, Sargatanas Jr. and Khalid are all good candidates.
1 Healer: If you don't have Ereina, you'll most likely need to use the axe beastkin as your main character's class so you can keep everyone healthy throughout the fight. I don't have Ereina so I use an axe main.
1 Dinari: Dinari boosts your critical rate
1 extra: If you have Drake, then your double-turn stunner is also a debuffer, which allows you to use an extra character. This extra character can be either of the above classes or just pure damage dealer such as event heroes.
Info for all strategies:
Formation: In order to maximize Dinari's effect, use a 3 row formation of 4, 4 and 1 heroes. Dinari should be the one lonely hero.
Fury management: Skills that require 300 fury should be avoided. They hit a 3x3 area and the boss requires to be hit at a single square. 300 fury skills deliver less damage than 200 fury ones. You will never be able to use all skills in a single turn - prioritize skills that heal, then those that debuff and then those that deal damage only.
Reinforcement: If one of your heroes dies, you'll want to introduce a debuffer. Bring in an attack debuffer if the death was a fluke; bring in an armor debuffer if you think you'll be dying the next turn so you can deal as much damage as possible on that turn.
Low or high-risk? All characters in my team have full SS judgment armors. I still prefer the low-risk strategy because of the lower variability in the damage dealt. The damage I manage to deal goes from anywhere between half a million to a million, depending on my luck . With the low-risk strategy, the damage output is stable around 0.8 million.
Low-risk strategy (recommended)
Round 1: If you want to make this fight last, you'll need to hit a 2-turn stun. This will give you breathing room to unleash your attacks. First, debuff the boss's armor as much as you can. You'll probably need to fury steal at this stage so you can then unleash the skills of your event heroes. You do not
Round 2: Hopefully, some of your armors' fury recovery inscriptions will have triggered, giving you even more fury to damage the boss. You should consider laying some attack debuffs in case one of your debuffers dies to smaler ennemies.
Round 3:Make sure you debuff the attack of the boss because it will hit you hard next round.
Round 4:Hopefully you'll have survived so far. Your stunner has 1 more round to go until he can stun again.
Round 5:Stun the boss again and follow the strategy from round 1. Once the boss is no longer stunned, he will kill your whole party in one shot with his skill. Its minions will clean up any remains
Round 6:Deal as much damage as possible
Round 7:Deal as much damage as possible
Round 8:Die. No exceptions.
High-risk strategy (for those who like to die dangerously)
Same as low-risk, except that you stun the boss on the second round instead of the first, which allows you to die on round 9 instead of 8.
An alternative strategy proposed by Foofaraw is to use Guinevere to protect yourself against the boss's skill. I haven't tested it but it has some potential :-)