Another tip to arafelis's stand still tip, try to kill enemies as close as possible to you to absorb their pixels unless you upgrade magnet first. This way its easier to level up.
This is game is indeed glitched or at least for me and a few other people. I have killed the mayor at least 3 times and no badge, i hope this glitch is soon fixed.
The only problem i see that isnt already said is that when you go to your upgrades and your holding down on your mouse (to shoot) but you let go on top of an upgrade you dont want and youb uy it anyway. If that was fixed the game would be much more fun, but great game anyway.
WARNING CONTAINS SPOILERS Guide 2
Right after the car crash go into the first door and just go right until you meet the survivor and durbin. After that leave and go back to the car crash but go to the left door this time. Check every room to find the survivors then go to the second room right of the entrance to find durbin. I highly advise buying an automatic here. After that simply just proceed to kill the bat again and you find yourself at the harbor. Just defend the survivors for 1 minute and then you find out they leave without you (dicks). SO then the end boss comes. This guy isnt too bad if you follow my tip. GO as far right to the bridge as you can and keep on pressing the jump button and you will propel into the air and he can't hit you but you can hit him. Make sure not to move right while in the air or you will fall off the edge and have to start over fighting the boss again. If this helped you please + to keep alive. Thanks!
WARNING CONTAINS SPOILERS Guide 1
My guide on beating this game is to immediately buy a chainsaw once you have the chance and grind until you either have 1.A decent gun with about 500 bullets or 2.A good survivor (I went with Bill). If you have bill you can just go into the garage and literally just stand there as Bill kills them for unlimited cash. But if not thats ok too. SO just get the car battery (its in the locked room right of the garage). And the gasoline is in the room that Frank finds on his own at the beginning of step 2. After that make sure you have at least 500 bullets in a good gun or Bill and proceed back to the garage. You will then fight a bat which isnt too hard as long as you have a gun or bill.
It would be cool if you'd get money to buy upgrades instead of it being just off of kills. I mean killing the gigantic 15 foot zombie and a little cat should not be equal. But other than that it's a good game, get achievements.
I know this is kinda a stupid lazy thing to say but whatever, I think there should be a radar because sometimes its hard to catch the tiny bubbles that blend in witht he bubbles you can absorb.
This may sound a bit stupid but I wish whenever you attack an enemy that one of you allies attacks and kills, you attack another enemy instead of just getting your turn skipped.
The best way i beat challenge 3 on epic was to max out all troops your putting into battle and max out invulnerablity then wait for 400 mana then just spam troops, dont use titans. Keep on using invulnerability so they cant even damage you and eventually your troops will kill baal then just storm the castle.