You can beat them with a single flaming sphere. They spend all their special attacks and invisibility potions for fighting the sphere because AI in this game is dumb af.
Larian thinks they can make the game harder by throwing more ac and hp at enemies but in actuality they make their own game more boring and unpleasant to play.
In multiple playthroughs, I've never had them focus on the sphere and not just break the caster's concentration and move on.
Not if the caster remains hidden and out of their sight. Once (on patch 4) I had a druid team up with Gale and just kept sending spheres over to the giths, while they kept to the shadows. Because enemies viewed the sphere like a normal summon, they'd get close and attack it, even though they could not destroy it. I have not checked if this was improved in patch 5 and if enemies now know to avoid the sphere. Imo Larian should just implement a way to get rid of summons in battle, perhaps a spell.
