Quantum Theory

Getting Intimate(20g)
Kill 100 enemies with melee attacks using the Bloodcleaver.

The Bloodcleaver is the "Claw Gun". The best place to do this(so far) is during the flashback(or flashforward?) where you play as the runt as part of a 5 man team. You will find this gun pretty early on in the level. Once you pick it up only use melee attacks. It will take 2 hits to take out the guys with blades for hands and only 1 hit for the rest of the enemies.

Also, when you and the leader of the team fall down a bridge and get surrounded by bug type things make sure you stay there and kill all the bugs. The leader says that there are too many of them and tells you to find a way out. Instead, just keep meleeing these bugs. They only take 1 hit to kill and they travel in groups so you can kill about 3 with one swing.

Make sure you dont just go running into a group of guys with blades for hands. If they hit you they will do massive damage and if one managed to clip you, the rest will too. Use your cover and shoot at them a bit first so you can go and melee them with 1 hit.

You should get this around where you meet back with your team and they start to plant charges on a giant seed.