When I leveled AST I was dreading doing this boss as I'm not the best healer, while you are right that it is unkind (I'd argue it's a little too much honestly) the damage is manageable. After healing the fight several times, I began to notice that the AoE damage has a certain ebb and flow to it during his casts that gives you some windows to safely cast and get people's hp back up, give it a few more attempts and you should start to notice it as well, it's hard to identify exactly when it is but if you do the fight more you'll see it.