Flare eats all your MP for a hard hitting AoE, but you dont need to be at full mana to use it. As low as roughly 10% is still enough fuel to start that particular fire. So flare at your level is mostly used to end an aoe cycle. You cast fire 2s until you have just enough mana to flare, then big boom flare, and transpose to get mana back. Using the Fire 2s means youve always killed enough time to get transpose back. It looks roughly like this.

Fire 3(full stacks!) Fire 2 F2 F2 Flare Transpose (as soon as you get your first mana tick)Thunder 2 F3 F2 F2 Flare Transpose Tick etc

You can also toss in a convert followed by a 2nd flare, and possibly a strong enough ether followed by a third - though both of those are on longish cooldowns and so arent usable every cycle. There are also a few points in the leveling curve where flare works as a final spell for your fire phase in single target, but usually getting a Bliz3 to fast regen ends up being more valuable for single target. The concept on the aoe rotation is that you always want to be casting a spell to hurt as many people as possible. The only time you stop casting is if you have to move, or while you wait the 0-3 seconds for the server to give you mana back after you transpose. Waiting for full mana isnt worth it(it only gets you one extra fire 2 and delays your next flare 10ish seconds per cycle, and that flare hits harder then three fire2s), so you want to keep pushing.