Early on, I think a successful rotation is:
1. Thunder
2. Fire (until MP is exhausted)
3. Transpose
4. Blizzard/Scathe (until Transpose is back up)
5. Transpose, and continue with Fire

Refresh the Thunder DoT at any point in the mix (or as soon as Thunderstorm triggers). Apply Sleep as the situation dictates. Mix in any other cross class spells you have equipped as you see fit, so long as you don't lose Astral Fire/Umbral Ice.