As with other strong mages, knock him down, stun him, and get him down before taking down the warriors around him. As is usual with mage enemies, Mana Clash does very high damage, although there is also a fair chance of it being resisted.It is vital to spread out, as his Fireball spell is extremely powerful. On higher difficulties, the boss tends to cast it at the start of battle before you can spread out; it can easily kill your mages and rogues before they can get back on their feet to heal. See: Dragon Age: Origins