Mindjack

To get this achievement you simply need to do a melee attack on 100 total enemies. The enemies do NOT have to be in a row. However, finishing moves do NOT count towards the achievement. You must knock them out with a melee attack until you see the "Mind Slave" prompt above their heads. Once you see that and if you continue a melee attack on them until they are dead that's a finishing move and it counts towards the achievement for doing 50 total, but they don't count towards this achievement.

Melee attacks are done by getting up close to an enemy NPC and pressing the B button when prompted to do so and repeatedly pressing it until they slump down injured with the "Mind Slave" prompt above their heads. Unfortunately doing melee attacks is kind of awkward in this game as you have to practically be lined up perfectly with them in order for the move to work.

Defeating 100 enemies using Attack Combos means knock them out using melee attack.
In this game melee attacks are not that simple as it sounds like. you need to be directly in front of the enemy and then start bashing but chances are the enemy will meele you easily. So the solution is

1.) Get another player as a red hacker. Let him mind slave the enemies and bring it to you.

2.) The above method is good but requires an extra players as well as extra time. so instead get this achievement in campaign itself using the simple trick. Every time when there are 1 or 2 enemies left when you are about to clear an area just run towards the enemy by holding and press when you are near the enemy. You will grab him as a shield. Now press and hold again to release the enemy. His back will be facing you and completely aligned. start pressing again and again to perform a finishing move.

* Enemy must be completely down or able to be mind slaved after the finishing move.

Simple way to do this is with a MP partner. My partner and I used scene 2-1 Panic Room. At the beginning of the level there are 3 available Red hacks, one below on the bottom floor and two above on the top floor.

Simply hack the Squawker on the bottom floor and walk to the top of the stairs. The blue player walks up stairs and gets combo'd by red and the A.I. will revive. Rinse and repeat until you pop the achievement.

also a good method for gaining exp. For the level 50 achievement, GURU.

hope this helps. It was by far the fastest method I've found so far.