Fireball until Heating up!
Cast fireball and infernoblast right when the cast finishes
Pray that both of them crit
(0 seconds)You have heating up and pryoblast! You start casting fireball
(2 seconds)You finish your fireball and spent your pryoblast!
(3.5 seconds)both spells crit, you spend your pyroblast, upgrade your heating up to another one and gain another heating up
(5 seconds)You having pyroblast! and heating up, you spend your pryoblast!
(6.5 seconds)It lands and crits, upgrading your heating up to another pyroblast
(8 seconds) you spend your last pyroblast and then both your infernoblast charges should be back