Breaching a grenade means you can hack a live grenade and prevent it from exploding. Grenades breached in single-player and co-op both add up. It is possible to grind this trophy in the second half of the campaign's final boss fight, the boss will occasionally launch an artillery strike of grenades at you where you can breach at least 10 or more of them with each volley. Its best that you drag the battle out and keep your distance in order for the boss to launch his grenades while playing it on a low difficulty. The only tricky thing you might have to worry about though is keeping track of the grenades when they're being launched into the sky.