Fireball (from max range), Ignite, Boiling blood, Withering Heat = normal mobs are dead. You can add Slow Boil befor WH too.
For single target PvP I skip any casting spells and use only instas:
Ignite, Boiling Blood, Slow Boil, Playing With Fire, Withering Heat, Nova. You can add Detonate or anything similar too.
Off course, this means you have specced up to Withering Heat (Immolation) and up to Nova (Icineration tree). Tactics: Ignition and Fuel to the Fire are welcome here. There are many different rotations for different spec too. Be creative
BTW,
Rain of Fire can proc Flames of Rhuin on multiple targets (because of direct damage).
