A couple of things:
1. The sniper has an insane rate of fire. I was able to continue almost indefinitely on Legendary with just a sniper and the next problem
2. Zombies don't jump down walls, so on the city map, you can run up the stairs to the 'L' shaped raised block where the medkit is, and kill of some zombies(Their AI gets stuck on the barriers), and then if you are under pressure you can jump down, and they have to go back and down the stairs to get to you, so you can kill them off and repeat.
3. Medkits spawn too regularly.
4. Burst rifle is OP, a large clip, a great range and an insane rate of fire means it is a great all-round weapon
(BTW this is Sam from REAP)