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I found a easier spot to do this, when you first get to the mountains after a helicopter ride( not sure in the level but early on, one past after getting off the roof tops) you come to a bridge that you will need to turn the power on to, so game tells you to go inside and there are 4-6 guys then once you flick the power to the bridge you drop down to go out and you see the small cut scene of a heaven walking in. So if you die or stuff up reload checkpoint and your in the same spot with the heavy and no other guys to worry about
I used the ak-47 and just single fired the body to I seen the armour fall off then I did the same to the helmet, and once both bits of armour is off punch him to kill him and achievement unlocks straight away