Melee-kill or stun 50 enemies.

To melee kill an enemy you need to press 'RS' during combat. This attack is quite weak though and not something you will likely use very often. To stun you need to be in stealth, and then press 'X' behind an unaware enemy to knock them out. If you're playing the game stealthily and especially if you're attempting "Full Strength" and its related achievements you will be forced to play the game stealthily so will almost certainly knock 50 enemies out without consciously attempting it.