Round 1: Boss is weak vs physical attack
Round 2: if he cast something that is 14 sec delay, def it or interupt it with celica's skill, i recommend def it though. Bring petrification nullifiesaccessories
Round 3: The main damage is from the heads, kill it. Bring 2 aoe damage dealers (celica is consider as one), 1 high single damage dealers like valefor to focus on main boss and 2 on def for reving. recommend 1 high physical defense and 1 high magical defense
Round 4: i do not know how to give hints for this one because i dropped it faster than it shoot laser

. it lacks damage but laser and the other long spell hurts alot so interrupt it when it long casting. use holy or fire weapons because of it attribute. Aoe Resist skill can be use if needed
Round 5: this one is quite balance, if it shoot laser, 1 of your char probably would be dead, it physical aoe also hurts a lot so ur caster would probably kiss the floor. on the bright side, it only has 150k hp so just burst it
Round 6: This boss got death proc and peirce. Bring death nullifies accessories if nededed. it defense sucks, especially physicalso just burst it. if death doesnt proc and you have someone on def, it is highly unlikely that ur entire would wipe out so just spam rev if u cant burst it
Round 7: This boss is actually alot harder compare to previous 6. He hit like a truck with only up to 7 turn delay and 7 turn cd. His spell can instantly wipe most of your team if not all so remember to have someone on def. luckily, this guy can also be super easy, his main damage is of fire and thunder element lol. so you just have to split your team in half, half wear thunder armour and half wear fire armour. cast rev after his aoe spells and kill him slowly. Done, nice and easy. Note, bring holy and fire weapon due to his attribute.
Round 8: I dont know why this one is round 8, it is difficulty is same as round1 and 2. It Has extremely low magic defense. It Hit hard, thats it, no proper aoe.
Round 9: This one can be a bit tricky. His damage is high enough to 1 hit your units, he also got pierce so dont bother stack defense and just go flat damage like using beserk rings unless you have confident n your defenses and hp to survive 1 hit. The most annoying thing about him is his 1 turn cd and 3 turn cast time for attack with chance of double attack !!! holy mother fucking god !
Round 10: Got death proc and use thunder aoe. Conclusion: use thunder armour + death nullifies. Done
Round 11: High magic Def and magic damage so use physical. Buff up and hug def. Aoe Resists is extremely useful here
All bosses from this point onward is hard
Round 12: aoe damage deal: 9999

. Stock up lots of aoe heal and aoe rev (25+). Spread out your team as 1 of his aoe is 3x3 . Always have 1 or 2 ppl on Def 2 opposite sides. Run some ppl with magic reflect for a bit of better chance of survival when he aoe. Run Action break lvl 5 can be useful. Cancel celica's action and cast interupt skill when you see him on a skill with more than 10 turn delays (forgot exact turn but i think it was 14). Have not test aoe resist skills since i think it wouldnt help
Round 13: Same strategy as above but this time focus on magic damage as it physical defense is high.
Round 14: This one is stupid, with 2m hp ,15% death proc, constant aoe, high damage, anything can goes wrong with this one... Same idea as the previous 2. But if you guys struggle against previous one, i recommend to stack lots more aoe rev and aoe heal items and farm some more level. Although i managed to clear this at lvl 195 on expert, it took me 6-7 tries to do so =< . Recommend use magic reflect, evasion and death nullifies to increase survival rate, pierce and chain attack to kill it faster
Round 15: mega upgrade version of round 3

. But this one rarely use head revival (i only saw it like once per fight when i test the mechanic for this). Similar ideas to the previous fews. Recommend 2-3 Aoe Damage dealers to kill the heads quickly. Useful skills for this: Fly, Evasion, magic reflect, physical reflect, pierce, chain attack.
List of useful skills for arena overall:
-flying
-chain attack
-pierce
-action break
-death nullifies
-binds nullifies
-aoe resist
-magic reflect
-physical reflect
-evasion
-negate all
-vital aim
p/s: i clear this on expert so dunno about the boss damage on normal