These are first encountered in the ‘Ruins Of Rekem’ section. The titans come out of the ground and wield a club/sword like weapon in one hand. To be able to perform an aerial slash on the titan you firstly need to get its life down to less than halfway, just do this by attacking it in any way you choose. When you see the titan kneel down on one knee you know it is time to do the aerial slash.
To initiate the move simply press when next to the titan to jump up onto it, then press . The prince will then jump up ready to strike, when you see his sword flash with a bright shine press again & this will finish the titan off.
I believe I counted 7 titans through the normal game play so you can miss 2 and still get the trophy. However, if you missed more than 2, don’t worry because you can get another chance at killing them in the challenge mode that is located in the main menu.