Use your Hero skills at the right time
Each of your in-game heroes has a different skill which can really change the tides of the battle and make the difference between a 2-star and a 3-star victory. Of course, in order to get the most out of the skills, you have to use them at the right time.
You also must know exactly what each skill does and – very important – how long it takes to recharge for a new use.
For example, the Punker’s first skill is the grenade which deals damage to a small area, but recharges quickly.
Squirrel has more powerful skill that basically wipes out the entire screen of enemies, but it takes longer to recharge.
So make sure that you use each of the skills of your heroes at the right time, when they deal the most damage possible to the highest number of zombies – ideally the ones that are toughest to beat with regular weapons.