Demolitions ExpertG 35

If you've already gotten the 50x for each of those three types, you will only need 50 more. For me, the M77 was the fastest way. Set up CTF on Rubble (1 flag wins so you can end whenever you want), and then get on your opponents roof. If you throw the bombs off to your left by the lot, this is where the booster will repeatedly die. They will spawn there every single time (because it's the farthest spawn from you) and you can get these super fast. Faster than the 3 grenades respawn.