I'm a huge fan of firestorm. With flashpoint passive (sounds like the one you over looked), you can keep spam firestorm and sometimes even stack 2 of them together giving you absolute lock down of an area. This works the best in Fereldan Castle for Perilous and above difficulties.
Barrier is always a good thing, no doubt. If you have Sceptor of Razikal you can probably even "tank" with barrier and fade cloak, at the same time stack multiple firstorms uninterrupted. Just make sure not to cast barrier when you are cloaked.
Wall of Fire is very handy as well, (so long as you are not facing a dozen of rage demons in NM) adding a bit extra damage and CC.
Strategy:
Start with wall of fire or barrier yourself, depending on the situation. Once you crit and see the flashpoint icon light up at the right hand side, cast firestorm. Ideally at chock points or on top of lots enemies, then run into the firestorm. By doing so you don't need to worry about melee enemies (except those immune to knock down i.e. pride demon). But do watch out for ranged attack.
You should be able to keep your own barrier up all the time because your hits will reduce its CD time. On the otherhand, if you have Sceptor of Razikal then even better. You should be able to trigger fade cloak often enough due to all the hits from firestorm. So you can spent a bit longer trying to barrier everyone else in the party over yourself.
As soon as your flashpoint icon light up again, cast firestorm. Don't waste it on any other skills. If you got hit a couple of times, don't worry. Just keep your firestorm up and your "death siphon" will take care of your health and provide you with enough mana. Just simply spam firestorm and everything else will take care of themselves.
Remember, more firestorms, the better. Because that means you are "safer" with everything near you knocked down and shorted CD time to keep up barrier or keep triggering fade cloak to give you invulnerability. If there is a horde of archers or despair demons, cast a wall of fire underneath their feet. But firestorm still takes priority.