Phase 1 is surviving. So a stamina trinket is a good idea. The cannon on top of the leviathan will cast napalm shell on you. It's easy to see because he will turn towards you and spark red for 2-3 seconds. When he does:
-use intervene after the initial hit
-or use deterrence before it hits
-or, if you have mirror of broken images, use it before it hits
To bandage, wait for plasma blast. Make sure you always have a pet CD up for plasma blast.
Once the leviathan is dead, phase 2 is exactly the same than in 10 man, just use intervene to absorb damage (and mirror). Make sure you kill it at most about 30 seconds from berserk. 10 is safer.
Phase 3 is surviving 30 minutes. Now assault bots spawn from orange beams every 25-30 seconds, and you need to feign death to reset them. However, you will need to kill 1 or 2 of them so that your pet builds aggro on the head. Misdirect all junk bots on your pet, and you should be able to survive the bomb bots. Dps slowly the head.
After 30 minutes, it's time for phase 4: kill a last assault bot, and finish off the head. Then clear all bots.
Phase 4: put your pet on the leviathan, and misdirect the head to him. Don't worry about your survival, as long as you aren't in a cone between the boss and the pet, and that the boss isn't completely on the wall, you should be able to avoid his barrage and outheal his hand pulse. Once the head is low (120-150K), attack the middle. Put him low on health too. Finally, attack the leviathan and, when he's near 100K, pop rapid fire and finish him.