it is pretty easy, as u may notice he heals when he loses 10k xp, So just before 10k is hit, Stun him (i prefer 2 turns) then damage hem more, so you know sure he heals(and your teammates wont heal him when u cast subv). Then when the stun expires, cast subversion, So that heal becomes damage, and damage heal.
It may take 1 or 2 tries, and he may heal later then expected but when he heals he is instant death. (2 million heal becomes 2m damage(i though he healed 3m with me))
I dont even consider it as a boss, too easy :D so i suggest u put like 2 iceboltthingys(the 1 that stuns for 2 sec) on ur bar 1 subversion. mayb an damage+ buff or heal, and high damage skills.
So u stun him first,
teammates damage him
he is stunned
damage him again
he is stunned,
Then just battle and mayb heal, till you'll hit 10.000 damage, Just before
991.000 (i think...) then use icebolt (doublestun) teammates damge him more,
He is stunned
Damage more,
He is stunned
Cast subversion ( ur teammates may damage him, so that becomes heal, mostly they atack efore you, so no problem.
He should heal now, if he doesnt just w8 he heals the second turn, just dont attack and skip r turn.
When he heals with subversion hes plain dead.
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-i think hes too easy for a boss
-it took me 1 try
Good luck and i hope i helped you, if you dont understand anything please ask, il be glad to help,
Cheers, skid ;)