Denny Ayard's video: Darksiders -- Final Boss -- Apocalypse Difficulty

Abaddon Destroyer Form, 2:25 : The key to this fight is keeping your eyes on his front claws and using Ruin to keep up with him. When he charges at you, one of his claws will glow. Move to the other of his body and slash him to temporarily knock him down. Dismount Ruin and attack Abaddon while he's down. Repeat this a few more times until he charges you with both his claws glowing. Press the Interact/Grab button when he attempts this double claw attack. Then, mash the Interact/Grab button when prompted to enter the second phase of the boss battle. Abaddon Angelic Form, 5:43 : At close-range, Abaddon can be pretty unpredictable, but his mid-range attacks have obvious tells that allow you dodge out of the way and then launch a counter assault. In particular, be sure in to get some attacks on him after he lunges at your or dives from the air. Three times during the battle, you'll have a sword clash with him that requires you to mash the Primary Attack button. After the first 2 clashes, Abaddon will teleport away and attempt to hit you with 3 large pieces of rubble. These chunks are easily dodged. After your 3rd successful clash, the battle is won!