Avoid killing other ulthork--killing them causes the counter to subtract one for each
Man-o-War will call two other ulthork during the fight - they die easy, and count against you! Watch for the red text, and don't use AE's after. Turn off AE-autoattack if you can, cancel all buffs and use Singular Focus. Run the adds off once the Man-o-War is dead. Ranged kiting can help here. Also, pet pulls work great for a Man-o-War in the middle of other ulthork.
If you have Feign Death (or anything alike), this will also work great. After killing the Man-O-War cast Feign Death. The two servants that helped the Man-O-War will disappear very soon.
It also might be a good idea to suspend your mercenary, if you use one, during this fight
The first time you kill it, it will regenerate to full health. The second time you kill it, it regenerates again, and Merrik gives you a poisoned ulthork spear. Use the spear as a charm - it casts a debuff on the ulthork that lasts 2 minutes. If you kill the ulthork while the debuff is on it, it stays dead