An assistance is basically reviving a fallen character. When they fall in battle, use the help option and let the mini game spin a few times until the two bars are blue. You get a good chunk of health, but more importantly, this achievement.
You can do so on a weak enemy. Just stand there and once it knocks a character out, knock it out but don't pacify. Instead you'll have enough time to pull off the assistance.
It's worth noting the fast forward system with LB is very bad at reviving and does it with mild success with just standard assistance, no way to let the game get it for you.