The key to getting this down is not spamming the grenade launcher too much. There are key parts in which you want to shoot in order to maximise the amount of time Nemmy spends in front of you.

The fight opens up like it normally does, run right, shoot low when you see his left arm go behind his back, grab the flame rounds. There will be fire in between you and nemmy, you're going to have to dodge through that (it can take some getting used to) then run on up to his front.

Update: Fire 2 shots into the heart, combine, then get off a 6 more shots to the heart and a flame round, Nemmy should now be below 65% and dropping.This should now force him to roar, his “cooldown” on being stunned from flame rounds expires after his roar so use this to your advantage, stun him and empty your shotgun shells into him, if you get chance try sneaking in another flame round here before he runs off. If you’re quick enough it is possible to get roar skip (you will have to use a mine round though to allow you enough time to reload your shotty; A flame round will not stun him if he is still burning, but you can recycle the stun by doing mine round to skip roar and then another flame round afterwards).

He now begins his 1st cycle, above the swat van, make sure to let him pass the first time so you can grab the ammo.

Update2: If his health drops below 40% in the first phase (which is possible) he will do what I call his tantrum (he stomps around at the ground). Do not recycle the stun, use this time to place a mine round/reload your shotgun/grab the ammo. Also you will want to adjust the placement of the mine to the bus, not the van as it will miss if you place it above the van in this situation.

If you follow where I shoot he will always land in the same spot, shoot a flame round at the ground near his feet (not too close) and you will set him on fire.... unload your 6 shells, reload, shoot him with one more flame round (before he gets back up). Nemmy will now have a temper tantrum, (this will be skipped if you got it in phase 1 and instead he will attack so priority on reloading the shotgun before shooting the flame round is better in this situation) stay close enough to shoot him a couple of times, use the rest of the tantrum to reload, fire one last flame round for a stun and shoot like mad. If you want to guarantee tantrum in after the 2nd phase, do not put his health below 40% in the first, then Nemmy will always tantrum after his first knock down after the wall (assuming you don’t shoot your flame shot too late to skip it)

P.S. It is possible to do this when starting at 8DA (see video below)