Im pretty sure the assasins creed event bugged the Heads Up! trophy. Kills from above do not count towards this trophy anymore. Can confirm beacause I've gotten more than 30 kills from above in this past week and I've played the game since launch and i stll don't have this trophy. The only thing i can think of is that the animation for the kill changed because of the event and it doesn't register as a kill from above anymore. Unfortunately there's no way to track kills from above which i think would solve this issue. If there was a stat that told you how many kills you had I would be able to show you but there is not sadly. Just thought i'd let you know.