First off, what level are you, and what difficulty are you playing on?
To help get you through that fight:
You can lower the difficulty
If your mage has barrier (which they usually should) then set it to a preferred spell in the Character Record > Tactics. That will help keep you alive.
Check to make sure you have your strongest weapons/armor equipped.
If you have a rogue in your party with stealth, you can climb one of the ladders, and hide there while your party fights, then if enemies come up, you go into stealth and wait.
You can just make your whole party run away the entire time.
The enemies come in waves, BUT you don't have to kill them all. They will continually spawn until the timer ticks down, at which point the barrier is destroyed and most if not all the red templars are killed.
After the barrier breakdown, you'll be going forwards to fight the Envy Demon. at the top of the stairs, just above where the templars are kneeling, there is a supply cache, so you can refill your potions before that fight.
