You hope that your talents aren't total crap compared to the ones the AI use. Like any tough fight in DA a good tactic is to aoe crowd control them and then bust down the casters while keeping the rest of them as cc'd as possible. 4 strong spells in this and most other encounters are: Sleep, Cone of cold, Crushing prison, Force field (use on a enemy player as a disable while you kill his friends).
Example fight: Wyene Morrigan Alistar Player (dagger rogue)
aoe sleep all of them before they have time to do anything, crushing prison on morrigan while everyone starts hitting her and nuking her. Move on to kill wyene while using cone of cold and force field to keep the rest busy, At this point it's a 4vs2 fight and it should work out fine.