When split pushing, position yourself on the place where you can throw Q on caster minions and it still affects melee ones, then hit melee minions with your passive and your sunfire will finish the casters. Also stand near caster minions while attacking tower.

Your basic combo is Q, E, W, AA but if you dont want to take risc you acn go for E, W, Q, AA. When you play agaisnt champions with cc like Camille, Renekton, Garen etc. If they enagage on you, use your W, it will get canceled by cc but it will proc aftershock so you wont take that much damage and you'll probably win the trade. Also if the enemy is in melee range you can cast your E behind yourself to get faster knockup.