Below are some easy ways to get some difficult challenges without boosting

Counter Claymore- An easy way to get counter claymore is to hide behind a narrow hallway or corner, like the underground passage below Highrise, put a claymore in front of you, and prone camp behind it. If it is a corner where you can enter from 2 sides, make sure your team spawns behind you, or you will be killed from behind. Camp prone right behind it, and when an enemy walks in front of it and you hear it click, then you immediatly shoot it and instead of going off by itself, it counts as a shot claymore. Danger close helps increase blast radius, so put that on for better results.

Hot Potato- Everybody cooks frags now, so to get Hot Potato, you get a class with a frag, Scavenger or One Man Army, and danger close. To get the kill as a thrown back grenade, throw it at the ground beneath you, pick it up, and throw it at an enemy, and it counts as a thrown back frag grenade kill.

Counter C-4- An easy way to get counter C-4 and possibly multi C-4, is to have a class with a C-4 pack, Scavenger or One Man Army, and Danger Close, and enter into an objective type game, preferably domination. Throw your C-4 pack on a domination spot, and camp the spot and make sure you're hidden well. When you see an enemy or enemies near the objective sight, then shoot it instead of setting it off by itself, and it counts as a shot claymore.

The Invincible- To me, an easy way to get the Invincible is with a class with a riot shield. Make sure your standing up when you see an enemy, let him shoot your legs a bit, and then crouch down when you start bleeding badly, and try to do that 5 times, and you should get it. It can be really difficult, because you might not know when to crouch before dying, but i would say it's the easiest way.

I hope i helped