seekers die in one good hit, eg middle mouse for mistwalker, or if they are unaware/confused and you get a backstab hit. Inside or outside her arena. In the arena, they are summoned in closer to you and maybe didn't reorient/retarget you and go into combat mode before you capped them, making them take the extra damage. If you somehow got them wet, water ups frost damage a lot.
fire and frost work well, as does just stabbing her. With the root chest and poison resist she is fairly soft hitting and anything works to chew her health down.