Kill all of the hedgehogs! Buy cats and upgrades! I recommend you should upgrade your income first, then buy a Khaul. Can you destroy the base of the Hedgehogs? Every time you destroy a bit of the Hedgehog's base HP, it gets harder! Start at Level 0. At Level 1 use the Pheno cat to fight the purple hedgehogs. At Level 2 you will start to see some flying hedgehogs.
The enemy base's HP is now 2000! Kambulacats are strong! Use them many times. Dynamites can appear and kill all of the hedgehogs! This happens when you are at Level 4 and beyond. This will help you in case it's too hard. More new stuff inside.