Did you try shoving them off the edge? Shove is a delicious mechanic. I have shoved so many EA bosses down long drops to their deaths.
Especially with thunderwave. Just don't put Gale too close to the edge because they tend to teleport to the bridge and will smash your mage with their 2H swords. Once shoved their HP is like halved and they are easy target for ranged characters or/and Laezel if she's still down there.
Mage armor + bless before fight starts.
