The two best spells you can get for dealing with mages are mana clash and crushing prison. CP is arguably the most broken spell in the game (it does Damage over time and paralyzes an opponent, enemies who are not paralyzed are slowed and still take damage). Use it on priority targets like mages and yellow enemies. A hidden feature of the spell is that if you use it on a target (friend or foe) that is under the effects of the Forcefield spell, they'll combo and release a large aoe damage and knockdown around the target. Just mind that your companions aren't in the blast radius. Mana Clash drains all a targets mana and deals damage based on the mana lost. Since mages have far more mana than health, they'll usually get one-shotted. It's also great for fighting demons. Another thing you should consider is bringing Alastair everywhere if you don't already. His specialization is Templar, which has some great abilities for taking out mages. He starts with a passive that drains mana whenever he hits a mage, and also has a dispelling spell and an ability that is basically Mana Clash with a different name. What i've found to be effective is to, in the tactics menu, have Alastair focus down any mages in sight automatically (unless you're the type to micromanage every single action). With his high health and magic resistance, he can usually take care of himself so long as you keep an eye on him.
Hope this helps!