On the lower levels, try to start with Blizzard 1 (for the Heavy - skip if you have a tank) and Thunder 1 (for the Thundercloud procs). When Thunder is active, go all out on Fire 1 till low MP then either go Blizzard 1 or Transpose into Thunder 1. Your best spell when in a group (and even when solo) is Sleep - especially when it becomes AoE.

On higher levels, you'll want to be doing something like this: Thunder 2 - Fire 3 - Fire 1 spam (+ procs of Thundercloud for Thunder 3 and Firestarter for Fire 3) - Blizzard 3 - repeat. That's the basic rotation and you can make it much more complex by adding Flares to the mix and depending on when you like to use Transpose.

The AoE rotation is Fire 3 - Fire 2 spam - Flare - Convert - Fire 2 - Flare - Transpose - repeat. Some like to add another Flare by using Ethers, but the damage output isn't that much higher since foes are usually dead anyway by that point.

There's a ton more info in the DPS section of these boards if you need it.