Warface

alright, so this was a tad bit harder than I anticipated, but it's not bad, and i'm going to make it even easier for you.

first of all, I recommend doing this against a heavy gunner (when you find one, a red bar will pop up at the top of the screen and it will say "heavy gunner"). second of all, make your life easier by doing this on one of the easier difficulties (note: because the missions change daily, the easy missions may not necessarily have a heavy gunner; however, they are pretty common).

i'd recommend having only one or two other people (unless you're coordinating with them). if there's only two of you, then one of you is guaranteed to get the achievement (unless you both manage to deal exactly 50% each).

you have, more or less, 2 opportunities to hit the heavy gunner DIRECTLY in that white circle on his backpack. the first opportunity: whenever he is distracted by your teammate and thus has his back to you. you can get a shot or two off before he'll turn around and mess you up. the second opportunity: when he is reloading. the SECOND opportunity is MUCH better! be a medic, have a shotgun, and you can run right up to him and pump 3 shots into his backpack before he reloads (again, make sure you hit the white circle on his backpack). rinse and repeat. i'd argue that you should just wait for him to reload everytime (it's somewhat obvious when he does, as it's one of the few moments when he's not shooting). heal yourself while waiting for him to reload again. stay close to him, but obviously have cover. rinse and repeat.

I personally killed him when I got the achievement; not sure if it's necessary to deliver the killing blow in order to get the achievement. let me know in the comments!

On one of the tutorial missions. i believe its 2 but not 100% there is a guy with a minigun in basically jugg armor. i was running the medic class so i had a shotty. just keep blasting him in his back. there is a fuel cell backpack which does critical dammage if its hit. i just self revived as my teamates hid.