On cargo you can camp in the building with shock charges, I hid and when I heard the charge ran out the closet to hit someone. Scavenge more shocks and repeat. Works in the top rooms of hijacked or in the hallways downstairs as well :)
Personally, I used Lightweight/Flak Jacket Perk 1 Wildcard, Scavenger for Perk 2 and (whatever) Mask for perk 3. Shock charges or Concussions for Tacs. If you don't use a lethal, you could Wildcard Perk 2 for Fast Hands or Coldblooded. With this set up, you don't need to worry about explosives (except killstreaks, RPGs or stickties) or tacticals. You can also close the distance quickly between you and your target. And finally, you can get back your tacs after every kill. General plan is to use your tacticals then follow up with the shield.
I also avoided HC. Personally, I found it harder due to them only needing one lucky shot through the shield to mess you up. (Can not count the amount of times I tried HC with it and ended up quitting due to being hit by random shots when they hipfired. =/ ) Sure, it's going to take two hits instead to kill, but you're not going to die as easily.
It can be done in most game modes with some degree of success. I'd avoid Hardpoint or Headquarters though, due to the fact you'll often have groups of people camped together, making it tougher to get a kill.
The best tactic is to hang around popular camping/sniping spots and kill people who are there (Windows in Hijacked/Raid, etc.). Due to the fact they're normally stood still, they're easy pickings for a tac followed by two bashes.
If you end up in a head on fight, back up around a corner and try to throw the tactical so that it gets them around the corner. Then move in for the kill.
It'll take a while to get those kills no matter how you do it.
After getting the 100 kills, the specific camo challenges are:
1000 score from blocking damage with shield - Simple enough. Just let enemies shoot the shield. You can get this while getting the others. 25 Hatchet kills while using shield - This means you have to get a hatchet kill while using the shield. Same general plan will work. Stun them then throw the axe. 25 Shield Bash medals - For this, the enemy has to shoot your shield and you have to kill them with the shield. Just do the same general thing as before, as most people will just shoot at the shield anyway when they see you. 25 Shield kills with no perks - This one's annoying. No perks at all. You definitely want to replace your letahl with tacs so you can take both shock charges and stun grenades. 25 Shield kills with no weapons - This means no lethals or sidearms. Tacticals are allowed, but you aren't allowed to replace your lethals with more tacticals for some reason. Weird. =/