THM in <35 dungeons (low level) advice?

[Rongway]

When using Thunder on trash, don't cast it on target #1. Cast it on target #2; target #1 will most likely die before Thunder ticks for its full damage. (Let the tank hit #2 or use Flash/Unleash/Overpower before you finish casting)

Besides that, the rotation without Fire III is

Spam Fire until out of MP
Transpose
Blizzard
Thunder or Blizzard
Blizzard (Not Fire!)
Blizzard (Still not Fire yet!)
Transpose

Repeat.

Breaking Umbral Ice with Fire I will result in two unbuffed fire spells (one to cancel Umbral Ice, and one to gain Astral Fire again). As it happens, the cooldown on Transpose is such that you can cast two more Blizzard spells instead before Transpose is up, and you won't risk shorting your MP refresh by breaking Umbral Ice early. Blizzard will do the same amount of damage as unbuffed Fire.

If the other DD is a non caster, switch to #2 when #1 only has enough health left to survive one or two hits. If you don't, you'll get interrupted because the other DD killed your target.