well im also struggling and went for the mk 48 instead (same challenges left (cherry blossom and stuff) i tried it on hardcore with:
Lightweight and ghost
Extreme Conditioning and engineer
with teh tomahawkzzzszszzszszs....
and i got the 100 kills so fast , now, for the other challenges, you may want to have a partner or two that use the assault shield as its COMPLETELY RARE to find someone using the riot shield, kill him, pick up his shield, and kill 25 people with it (im leaving this as the last challenge)
I did it mainly by playing domination. Taking B flag (flak jackets and tac mask absolute must!) and then sitting near by. You'd be amazed how many players will just dive onto the flag with checking their surroundings. And then for the smarter or more observant enemies, shock charges to hold the buggars still while the bashing is unleashed.
Eh, I can tell you at least how I run it. Though in the end it's mostly based on a mixture of individual skill and luck. Running with a good team helps, also, especially if they know that you're trying for your kills and they won't steal them.
Scavenger + Fast Hands
2x Shock Charge
Wildcard - Perk 2 Greed
I've found that with the connection issues the game has, along with the fact that the shield is severely lacking in lunge ability, it's not worth even trying to get to people most of the time. I tried running lightweight/extreme conditioning, and it just doesn't work because they still move faster than you walking backwards firing an LMG and half the time you just get stabbed through your shield for your efforts.
You need to make them come to you. Use your shock charges at corners and draw the enemy into it. If they start running away or you face multiple enemies, just use the b23r. As always with the riot shield, try to wait until you see an action animation begin before advancing, and if they're standing still while doing so, use the opportunity to kill them with your axe.
I got mine diamond yesterday, this is what I used -
Mostly I played hardcore KC and had to be very stealthy about it
my class setup was -
Scavenger/fast hands with perk 2 greed for stun grenades
Extreme conditioning/engineer for seeing Betty's etc
2 stun grenades
You best bet is to try and jump on them, stun them as much as you need and then try and get behind them