How does a Fire Mage convert Heating up without Fire Blast procs?

[DisRuptive1]

And also no Combustion, no Dragon's Breath, and no 2-piece set bonus.

Do they use a Phoenix Flames instead?

[Gestopft]

Well outside of Combustion, Phoenix Flame will do it if you are talented into Alextrasza's Fury (same node as From the Ashes). If the enemy is below 30% Scorch will do it (assuming Searing Touch). Otherwise, you just have to rely on RNG