Syndicate

Last mission
When you defeat two minions of the last boss, he will jump off the elevator and throw a lot of grenades, you just need to press RB to slow mo and defuse grenades in the air...
You can get about 100 in 15 minutes - very easy achievement
Just not shoot at him, pick easy difficult and add all points to health and health regeneration.
Good luck

There's a much quicker way to do this, takes about one hour to breach 873 grenades.
Make a private coop match on Expert in Scandinavia, use a sniper or the Kusanagi. Fight your way past first checkpoint. Shoot the riot shields through the wall with Dart Overlay, but don't kill the guys on top of containers. A container might pop some guys on the floor, kill them too. The trick is when you come out of the checkpoint door there will be some wooden crates on the right side, sit down with your back to these crates. The guys on top of the containers will throw grenades at you. It will be raining grenades, just breach the ones that lands in front of you, there's 4 guys throwing them constantly so just breach away.

heres my video to getting it. 8) Takes 30 minutes or less.

This method is much easier, but it will take you slightly longer to get the achievement... Load up chapter 15 on easy and as soon as you drop out of the air vent, crouch behind the crate, turn 180 and face the propaganda poster.... Make sure you are in the corner between wall and crate... There will be 2 guys behind cover that won't approach you and they will continually throw grenades towards you.. They will throw the odd Molotov as well but you shouldn't get hit by them. Simply breach the grenades as they bounce off the wall.... Sometimes they might fall short, so just keep your eye on where they land....

If you are just getting bombarded with Molotov's and hardly any grenades, just restart checkpoint and try again. Took me 2 and a half hours to breach 873 grenades.... If you get bored and want to save, there is a checkpoint just past the turret and through the door up the stairs.....

I'll throw this one in:

Load up Single Player Chapter 15: Kris on Easy.

Spend the points on whatever increases your health and the duration of DART.

Make your way through a few enemies and get to the battle with Kris. Keep shooting Kris until his health is at the halfway mark. This will trigger a cutscene where he detonates all the EMP mines around the room permanently and then beats you with his gun. You must Breach/take his pistol.

From this point on, Kris will will run around the area and only throw grenades because he doesn't have a gun. He turns invisible, but he's pretty easy to spot. His usual pattern is to throw 2-3 EMP grenades, and if those successfully take out your DART system, another volley of 3 frag grenades.

The best strategy here is to keep relatively close to him while also keeping an open space behind you in which to back up when he tosses the EMP grenades. Keep in mind that you have to be able to see the grenade to Breach it. When close, he'll usually toss them at your feet, so you can hit RB and go into DART and while backing up, immediately press LB on all of the grenades in the cluster (you need to press and release LB with each grenade). If the EMP grenades take you out of DART, run away from wherever you're standing because 3 frags grenades are coming at you and you can't Breach them. Even on Easy, getting hit by all 3 will likely kill you.

The drawback is that while he sits in some spots for a little while, he never stays in one place for very long. So you still have to put in effort to move around to keep him maintained at the perfect distance.

It takes about an hour to do depending on how many grenades you've Breached up til now. This strategy is less stress than the other solutions suggesting soloing Co-op on Expert, taking down Merit's bodyguards or dealing with him and his gun shooting you constantly.