Stand further away. Trust me. You can for some reason hit him way more easily with more distance. And the max distance allowed on this part is jarring.

Also, the flank and back on Titan's heart seems to be dependant on who is aggro'd on. Stand across from the person with aggro for the back attack, and perpendicular to them towards Titan for the flank attack. If you do all of this you'll ensure your combos go through with fluidity.