Assuming you aren't talking about the Combustion burst phase, you chain cast Fireball. Never stop. At some point you'll get a heating up proc. By this time you'll already be casting your next Fireball. That's ok. Simply press Fire blast. You can cast it while casting. Then, as the Fireball cast ends, fire off the instant Pyro too. Then hopefully one of the Fireball/Pyro crit, and rinse and repeat.