Pikotaro is a Metal enemy, which means he only takes 1 damage from all attacks except for Critical Hits. Since his attack rate is quite slow and his stage does not have Assassin Bear to force lose the player when they takes too much time, using fast-hitting cats is an effective way to deal with him. Even spamming Eraser Cat is enough to kill him.
On the other side, Pikotaro has very high attack damage which can kill most cat units in one hit.
Metal Cat can be helpful to stop Pikotaro from getting near to your base.