Mages are very good at destroying large groups of weak or medium enemies but less so at killing high health single targets because there isnt really a strong single target spell with low cooldown unless you count spirit blade. Your sustained damage output is generally going to be far less than rogues.
Its worth nothing from personal experience that riftmage and KE are vastly better than necromancers.
Also, CC in DAI is nothing compared to what it was in DAO.
Try Having solas start with pull of the abyss, drop a fire mine at the center and as soon as the mine explodes and gives you full mana, follow up with a static cage then chain lightning, then viel strike(this will cause deep sleep combo on most enemies) then immolate(sadly bugged and wont do an eldritch then wall of fire. Most humanoid mobs not immune to fire will be dead or near dead by this point even on NM.
If you have decent crit, then you will likely get a flashpoint proc somewhere in there allowing you to cast either pull or cage in rapid succession without needing to wait for cooldowns.