Are you knew to the game?

The blue flame emitting from Aku is actually a shield, only some Aku enemies have shields.

You need to break to shields in order to actually damage the unit.

The shield will come back with full health if the enemy get knock back.

There's an ability called "Shield piercer" that has a chance to break the shield immediately. Some cats have 100% shield pierce.

Aku shield is different from Star Alien shield.

Starred Alien's shield(barrier) only can be destroyed by the "Barrier Breaker" ability and damage that is higher that the barrier's health, the "Shield pierce" ability doesn't work on Starred Alien barriers.

The Star