Pikotaro is a Metal enemy, which means he only takes 1 damage from all attacks except for Critical Hits. On the contrary, Pikotaro has very high attack damage which can kill most cat units in one hit. However, his backswing is also long so stalling him won't be an issue at all.
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 Tank Cat or Cow Cat is enough to kill him. Metal Cat can also be helpful to stop Pikotaro from getting near to your base.