Attack stages
Move forward and backward in a straight line. As soon as you reach the end point, tap the bottom of the screen to make a 180 degree turn and start shooting at the zombies that are moving towards you. Destroy them as fast as possible so that you can move to the other end of the line and get kill bonuses in the process. Once at the other end, turn around and repeat the process.
Defend stages
There are three points that must be protected, and they might not be placed in a straight line. If the zombies break in, they will all come at you. You can shoot them at point blank range.
Multi kill bonuses
You can get the multi kill bonus, especially in attack stages, by shooting each of the zombies a few times, but not killing any. After all the zombies have been hit a few times, finish them off one after another.
Scoring
Try to get as many headshots as possible. Headshots will earn you more points and fill the Frenzy meter faster. Frenzy mode gives you a lot more points. To fill the meter, avoid being hit by zombies, get headshots, and kill ranged zombies.
Straight Lines
A good strategy in this game is to approach Attack stages moving forward and backwards in a straight line. As soon as you reach the end point, tap the bottom of the screen so you turn 180 degrees then start shooting at the zombies that have started advancing towards you.
Various Game Tips
Don't forget to actually collect the kill points zombies drop when they die.
Get those multi kill bonuses
Always upgrade weapons because upgraded weapons give you better damage.
Basic gun goes all the way to level 25, so don't be shy to take it to at least level 10 (and replay the stages with it upgraded for a real blast!)
Always try to hit as many headshots as possible they give you more points and fill the frenzy meter faster.
Frenzy gives you a lot more points to fill it faster, get headshots and kill ranged zombies in order to fill that bar as fast as possible.
The best strategy to approach Attack stages is to move forward and backwards in a straight line - as soon as you reach the end point, tap the bottom of the screen (180 degrees turn) and start shooting at the zombies that have most likely started towards you.
When approaching Defend stages things are a bit more complicated since there are 3 points that you want to protect and they might not be available in a straight line.
If the zombies break in, you don't lose the stage or something: they just all come at you and you can shoot them down from point blank range.