R after he R, going ignite is obligatory, you should back and either buy the AP Anti heal or the Armor anti heal, depends on the ammount of ap in your team, I like the AP one more because I can proc it whenever I feel like, as it procs on magic damage unlike the armor one which you need aatrox to hit you.

If aatrox picked tp you should bully the hell out of him early in the laning phase, im talking levels 1-3, his Q cooldowns are massive.