Seeing as the description tells you nothing, you have to get 100 kills while staying on the wing of the plane (located by Juggernog). IF YOU LEAVE THE WING AT ANY POINT, YOU HAVE TO START OVER.
I did this on round 6 and started with the XM4. It only really got hairy after I ran out of ammo. When I did, I started walking circles on the edge of the wing and threw my tomahawks at zombies. This still counts, as you don’t have to sit at the top (although this is the best spot since you can’t get hit from behind). I finished the 100 kills by the start of round 8 and I did it solo.
If you downvote, please explain why so I can fix the solution! Happy hunting!
EDIT: according to comments, you can leave the wing and return (as of right now).