The trick is, at least for me, to never ever get anywhere near Aizen and just fire projectiles at him until he is stunned and finish him off there and then. He's an absolute pain in the tush when he's alive.
From the flying, homing rockets to the black boxes to that infuriating life-draining grab. It's crazy. Also, when he's not stunned, he teleports away from every meele you do when his health is less than half.
I used Skullclad Ichigo on Normal while I was like lvl 60 or something. Anyway I just focused on Aizen first while using projectiles while keeping a distance and if he tried to pull me in I just jumped into the air and tried Dashing in the opposite direction which saved me by 4 seconds at the most. Then when he reached a little above half health I used Ignition mode to increase my strength and to spam the three skulls in ignition. Once he was really low I lined him and Ichigo in a line then fired the Hell Getsuga Tensho then just focused on Ichigo til he fell and avoided him when he was in Ignition Mode.
PSN: KiraYamatoStrike Medics in Bad company 2 are the ****ing devils, yep I hate them that much.