The boss on the first map (I think) of Act III is wrecking me. Every so often he just oneshots my entire party. I'm dodging all the ground-marked AOE and keeping spread out so I can rez in case I miss one. But yeah, like I said after a while everyone in the group drops dead all at once.

What am I missing?

Couldn't hear the audio on that video, but found another one that helped. Apparently I'm supposed to stand IN the green circle.

I also learned new things about the interface from it, so that's good too.

Seems like you found the answer already, but just be aware there are a couple other bosses you'll face that have similar abilities. When facing a boss (or a new enemy, for that matter), it's a good idea to pause the game and take the time to review the descriptions for each of their abilities so you'll have an idea of just what they'll be trying to hit you with. Makes countering and dealing with the after-effects much easier. I remember towards the end of the game I started getting wrecked by certain enemy groups until I did this and noticed they were hitting me with an ability that turned any healing I used into damage. Lots of interesting abilities in the game, and a big part of combat is making sure that the enemy can't effectively make use of those abilities.

Yeah, until I watched that video I had no idea you could read the enemy abilities on that status window.
Just that was a big enough help that I finished the game the same day I beat the Hydra.