The wiki says it's unlocked in field E6 and another source says Q6, but I've cleared them both multiple times. What is the definitive way to unlock this battle trait? Was it changed in the latest upgrade?
i don't remember which field it was on, so i can't help there, but when you're acquiring new skills, you have to unlock the "skill tome" on that level. skill tomes will be squares with circles in the middle on the field, and when you mouse over them it'll show a range circle and a tooltip that will say "kill <number> <afflicted> monsters/swarmlings/reavers/giants within <range> to unlock"
so to get the skill, you have to kill <number> amount of the listed monster type within the radius to get the skill. occasionally it will call for cursed, frozen, or cursed AND frozen monsters to be killed, in which case, hit them with the correct spell(s) before killing them for it to count.
My notes tell me, I have unlocked it in Q6, together with T hextile, U hextile, T1, U1 and The Forgotten... I can't recheck at the moment, maybe later... The only slots I haven't it unlocked are my "f-hextile"-slot and another, "low-level-low-skills" - at that slot I stand at Q2 but could not beat it with my settings