The ultimate is always optional, use Phoenix when need a zoneout otherwise use the Pyroblast.

When U have a team that helps u to stay alive and watches over U, U can gather the stack for the Flamestrike build.
BUT if U don't, U should pick the other build, with a different tactic.
Use trait for living bomb mainly. Cast it on targets that about to die, so it will spread immediately.
Your trait is also valuable if you cast Gravity Lapse on a single target because of the longer stun
If U R against a team that loves to spread the living bomb, than use Master of Flames on lvl 20.