Endgame rotation changes a bit once you get the teir3 spells as they overwrite the umbral or astral buff. The rotation that works best for me (not sure if its the best, but welcome to suggestions) is:

1.Open with swiftcast Thunder 3
2.fire repeat with fire3 when firestarter procs. (also tossing thunderspark procs here as well)
3. when below 500 MP blizz3 (overwrites astral 3 with umbral 3) also blizz3 (all blizzard spells) is half mp cost and half casting time when under astral 3
4. if swiftcast is back up i reapply after getting back to umbral 3 otherwise i toss a scathe in while waiting for umbral to get mp back which takes 2 ticks (6 seconds).
5. at about 5 seconds i start casting fire3 and by the time the game registers it lands and overwrites my umbral3 with astral the second tick to top me off has landed.