This is how I made it past multiple waves playing 2 player horde mode in the Canyon map. It took many hours to get to Wave 60 but here's the tips on strategy that I recommend:

1. Aim for the head.

The less bullets you use early in the game assures that you will have ammo later when it counts; therefore, take your time and aim as much as you can. There will be a pace later in the game where aiming will put you at risk but until then develop the aim.

2. Barrels

Shoot the barrels when you can kill 3 or more zombies. This saves bullets and speeds up the rounds with multiple zombie barrel kills.

3. Burgers

Save the burgers when you get to the round breaks. Every 5 waves there's is a break.

Hold burgers until the round restarts and they will remain. If you have too many burgers to hold then take bites out of them as partial burgers will remain in the next round. I have not yet figured out why this is but it seems to work about 80% of the time.

4. Ammo

If you are full of ammo, leave your partial clips for the next round. Shoot a couple of bullets and then drop the clip. It will last until you need it later in the game.

Balance your ammo use - for example, if you have more sub machine gun ammo, use that until it's below the the next reload quantity. At some point your ammo will be low enough that it won't matter but early in the game it will help to use equal quantities so you can reload all of the ammo that has spawned. Additionally, this gets your to practice aim with different guns

5. Health -

While some think this is cheating, it works in 2 or more player games.

If your health is low let the zombie's kill you. Have your partner kill the zombies and you will respawn with 100% health. Great way to get your health back without eating a burger.

6. Weapons

I like to use the colt and the deagle mostly because of the accuracy. The colt has the power and the deagle has more bullets.

The Colt was the weapon that "Won the West..." so it makes sense that it would be beneficial to getting past multiple waves. I keep an automatic sub machine gun or shot gun in one hand and use the colt in the other. This allows me to aim and when needed blow up a horde that has gotten to close.

7. Runners

Shoot the runners first. This goes without saying but it may not be obvious to the uninitiated.

8. Flares

Save the flares until such time that you need a break from the hordes and save your ammo to the next round reload.

Throw the flares to a position where you can kill the largest Zombie horde with grenades. So you will want to make sure you have some grenades (at least 4-6) to kill the most Zombies for the effort.

9. Graphics and Glitches

It seems that in higher numbered waves (after 40) the more quirkiness the game experiences. I turned down the graphics to average so that I could avoid the glitches.

Some examples: waves that don't end but there are no more zombies to kill. No way to exit other than quitting the game.

Guns that float or stack on top of each other. This is something I've experienced quite a bit. I reach in the stack and any gun will will come out. I can't pick the one I want because they are melted together.

Anyways, hope this helps others. If you have other tactics to share please let me know.