The easiest way I have found to do this without any combos/attacking was if you can single out an enemy with a shield, i.e. clear all the other enemies but leave one that has a shield, just keep spamming the B button. It is the grab move and it does count as a combo even though the shielded enemy shrugs it off. This will also keep the enemy from attacking you as well.
The way I got it was when I was on Mission 4 with the Zamaron's. I had two shielded enemies and just kept alternating the grab attack on them but singling out till there is one while doing this makes it much easier.
Keep pressing B button until x99 and the achievement is yours. It may be cheap but if you are having trouble, I recommend this.