Pryoblast will actually slow you down while grinding 2 mages.
For grinding fire mages try this. I've found it does a lot of fast damage but is still pretty mana efficient, so you won't have to stop and drink as often.

With 2 mages at max range,
fireball > scorch > fireball > fire blast (if needed)

You can get off the first 3 in nearly the same amount of time as just a pryoblast and the mobs will be dead already.
Most of the time fire blast won't even be needed. It's more of a back-up if a malee mob has a chance to reach you.

I use this for lots of fast single mob kills. If there are two mobs, they both get sheeped before I start this on one.
For 3 mobs, sheep 2 and start this on the third. The third will likely reach you, but die quickly afterwards.