Gamer has been removed
Okay since the other 2 solutions are strange and dont explain exactly what you really have to do:
Go to any enemy which you can grab by pressing the "RB"-Button and hold the "RB"-Button (On some enemys you have to do some damage, before they let you grab them) You can do this in combat and aswell in stealth-mode by sneaking behind to them.
Now all you have to do is press the A button while grabbing them, which will brand them and gives you a count for the achievement. Killing them by pressing the X button will not count toward this.
repeat this 99x.
Ofc you can also hold the B button while grapping them, but i wouldnt suggest that as it takes much longer.
sorry for my bad english, but i hope you can understand it.