100% Achievement

Easiest way to get the PRO achievement

PRO

The achievements are: kill 1, 50, 250 zombies, get 50 headshots, 10headshots from the crossbow, kill 10demons and reach round 15.
The first 6 are pretty straightforward, demons come around stage 10.
The easiest way to reach round 15 is to play The spiral map. Go to the corners and shoot the enemies(most of them will come from above). Then go up with double jump to the next corridor and kill those enemies. Clear the first 5 stage then start going to the top. You need to click very fast on stage 6. When you reach enough points spend it on the 25000point weapon. Finish the remaining enemies with it, then buy ammo. You get 100 for 3000points.
If this is too hard for you buy the 5000point pistol first, and buy the 25000point weapon a bit later.
Repeat what you did with the pistol. This weapon will instakill all types of enemies and since a lot of them will be stucked on different corridors you dont need to take out all of them at the same time. Buy ammo at the end of each round. You can leave some enemies on the bottom floor, that way you will have enough time to go to the top.

[CokeMan]

Worked out perfectly for me, if you have the 25000$ Blade you can camp on the lowest spot on the map and wait for the enemies. Make sure you have much ammo for the rounds 12+ so you don't have too jump to the ammo station during a wave on the top.