Ok so I'm sure you've seen this achievement but didn't understand it. Bet you thought you had it already. WELL, its got nothing to do with the flame wall, its lava strike.
An easy way to get this is kill yourself early on in the battle.
I'd suggest using a spell to taunt the boss, get one shotted and then say "oops, wrong button, sorry" or stand in the flame wall + jump in lava, hit by tail etc.
Then, when your group kills sarth, you get the achievement since you dont get hit by lava strike (a random aoe spell).
Thats all![]()