Tips

-Don't be afraid to sit in the middle and let people pass you
-Hit detection is great so don't be afraid to try to dodge people
-Flamethrower is overpowered, use it until you get the gun after the rocket launcher
-You can headshot with flamethrower and its very easy to do, just aim up when your beside somebody
-Chainsaw is useless. Stronger then the flamethrower but has a shorter range, meaning you get hit much more
-Laser gun is the last weapon and will basically let you play levels 30-40 without getting hit

Medals

Just going to touch upon some of the hard ones. The majority of medals you get for just playing through the game (like die or reach level 20 or kill 1000 people).

Collector (pick up 10 pickups in a single level - 10 points) - For this one I would try to stand in the middle of the screen and watch the right side of the screen for items to go buy. They all count (the poker chip looking things, different sized stacks of money, and the little black things). You will probably get it on the first couple stages. If not, wait until you have the flamethrower, which will make you less likely to get hit while looking for items.

Freeman (Kill a big zombie with a crowbar - 10 points) - Very easy to do. The big zombies are the grey ones, can't miss them. I used flamethrower to get his health low, and then pressed 0 to switch over to the crowbar and hit him once. You can also just take out your crowbar and beat him and hope he dies before he hits you 3 times.

Boom! (Kill 10 zombies with a barrel - 10 points) - Going to assume this means 10 zombies at once, as I didn't get it and I killed probably 100 zombies from barrels. Also i'm going to assume you have to do this with the barrel and not the box of fireworks (found in the later levels). What I would recommended is to use the flamethrower to get everybodys life down to less then half, then dodge around and wait for a barrel. Must be done on level 18 or further because you wont get 10 people on screen at once until then.

Shredder(Unlock the chainsaw - 25 points) - You unlock the chainsaw after getting a certain amount of points. You should see it unlocked somewhere around level 24-29.

Zoller - (100% headshots in a level - 25 points) - Easy to do, just stick with your first gun on the first level and only make headshots. The level is 15-20 seconds long, so you can make 6-7 headshots, then dodge for the rest of the level and get it.

Sureshot! (Get 100 headshots - 50 points) - Like I mentioned in the tips, very easy if your using the flamethrower (or sniper if you prefer). Just aim high, the judging system of what is and is not a headshot is pretty good. Odds are 2/5 kills are headshots, so you should have that one done by level 23-26.