First it is good to note that I think I'm nerfed on some zed(zombies). So I'm not sure if this guide is totally accurate.
This page could be useful to understand which zed I'm talking about:
Let's start by the most common zed, the clot. The first wave almost only contain clot and the other wave have some too. Here a picture of this ho: http://kf-wiki.com/wiki/File:Clot.png
So they are easy and the weakest zed on the game. But they take ammo and time and they can sometimes be annoying or should I say dangerous when others large monster are needed to be kill quick. In the first wave, they are easy and ammo management is not really important for a good gunner as me but could be for normal gunner. So you can just aim anywhere on it in the first wave to kill it. If you want to save ammo, you need to shot the head first than couple of shots on the body. If you want to be real good like me, you need to get real near of it and than with the 3 shot starting riffle aim for the aim without ads and shot the first shot in the head and the rest of the burst in the body. This technique is great and work great but you need the good angle. The clot need to face you to do that. I rarely succeed to do that when he does not face me. In the superior wave and hard wave and at the difficulty I'm playing, winning time and saving ammo is a must to pass the wave.
Edit: Note that it appear that for some reason you can only shot the head at very close range and get the kill like if you have shot the head once and the body once too. It's God that made me write this, I was not aiming the head first than the rest of the burst on the body. Now I do it but I was not.
Than come the other that look alike it(slasher and the cyst). Check out the first link I posted in this post. Same technique can be use.
Ok now the Crawler. This one is easy. It require I think 2 bullet of the starting riffle. So again if you mind about ammo management and time spare for less reload and less time on it, you need to aim with ads and shot just a little burst. A great gunner like me does not need ads to kill those...
The bloat: Ok so this one is only dangerous if it get too close and he is slow. The right thing to do with this ho is to first get rid of his head so he loose what of his capability, the green shit trow. With no head he can still hit with his blade but he is slow so. In a kaos battle, I take out his head as soon as I can and than get rid of the other more fast and more dangerous zed. Than I come back and finish this fat ho.
Edit: It appear that if you take out the head you do not need to take out the rest of it, it die after 5 meters of walking.
Ok this one is like the clot somehow but he got blade and I think he is quicker. The same technique as the clot can be use but don't get too near. If he start to attack than most gunner I guess need to forget the head and shot anywhere on it.
Simple, just aim the head until he blow up than finish it on the body.
Ok so this one can shot at range. If you see a ball of fire starting to grow on it, you better run out of were you are. Than you shoot it in the head until it blow and than finish it with some shots on the body.
SCRAKE: OK this is our priority target. Everybody quit aiming on other target and shot on this one. If you can when he is far, you can do a lot of damage in the head but if he start attacking and spinning like mad, it appear that aiming the body is more efficient. A good team player and a good team center fire on this one as it appear. If this guy is not kill he can easily wipe out a entire team. I got kill several time by this one.
FLESHPOUND: Same as the Scrake. But this one is not as slow. He should be kill and hunt before the Scrake. Of course if we start to shoot a scrake for a lot of times and he is almost dead, better finish it before starting to shot the Fleshpound.
Ho yea and keep the grenade for the Scrake and the Fleshpound imo. Sometimes you can easily hit couple of grenades because they don't move because they are attacking or stuck.
For the BOSS. Well ok so everybody shot at him. The smart players will figure out how to stay alive. Use the nade when he start being red colour. Aim the head if you can but if you miss to much, rather aim the body. Be careful, he shot at range, so run when he start saying he shot stuff, yellow and red stuff. A good team in require to beat this beast at my game difficulty. Ho yea and don't forget to flew and heal if your hurt.
So this is it, I'd finish this by it is always better to stay in large team(whole roster) and some area of the maps appear to be better than other to take the wave. A clean and large field is greater imo. Closing door could be useful, I'm not sure of this.