ANARCHY REIGNS

Just before playing that mission, do the one with the mutants but make sure you have no item. The mutants drop a missile launcher and the supermutant drops a rampage item. Keep it, kill some guys until you have charged your rampage and start the final mission. You can now go berserk and when you are no more in rampage mode, use your item. It should be enough to execute Cthulhu in less than 5 min.

you can get this achievement during black side stage 4 on the main quest Butcher's Banquet. not sure wat the name of the quest is for the white side but once i find out i'll update this.
playing on easy mode will make getting this achievement alot easier.
u want to do is get your rampage bar all the way fill so u will be able to rampage during the fight. when the mission starts u will the choice to choose between, jack, rin rin, and big bull.
i chose to use jack for the fight. when the fight starts, cthulhu will have purple shield that u will have to break during the fight. once u break his shields he will kneel, when he does activate your rampage and start using it on him. if u connect u will do alot of damage to him.
the camera angle might give u some problems. also if u have a rocket launcher use it in the fight it will cause a great amount of damage as well. when cthulhu runs out of health u will have to do a execution on him by pressing b to finish him off. once u finish him if u made it within the time limit the achievement will pop for ya.