No single AOE should beat your focus attack. You have 4-5 guys focusing attacking him. And he is just one guy. Eat the AOE and take him down. Don't run and try to avoid the AOE. You are NOT avoiding any damage when you run. and on top of that, you are not doing damage to him.

I would assume he gives you trouble because he runs around you avoiding your casted spells. and at the same time doing holy nova, or arcane explosion. For that, it is a facing problem. Frost shock, earth bind him if you have it. Then practice your facing.