The trick to the titan slam on bosses is they MUST use their melee attack before you get within range. Phogoth is the easiest for this, just jump on the raised platform and jump/hover above him until he smashes, then super him. He will smash nothing (because your hovering 50ft above, therefore out of range), then you will come flying down on his head, at which point you turn and sprint/jump back to cover. Easy.