This is a pretty self explanatory achievement.
Get 100 melee kills with either Iron Man or War Machine. It can be done on any level or against any enemy.
To melee, you hit X. You can chain together moves and combos, for a list, hit start and check melee moves.
There are a few good places for this.
Level 6 has tons of small one hit drones and an infinite spawn at the end. Also, level 4 using the same.method most use to farm weapon kills.
If you combine this with getting 50 melee kills whilst Invincible as Iron Man, that will gain you another achievement.
Hope this helps, any questions just ask.