Fun game! A few areas to improve:
1. Each enemy with unique behavior should have a different appearance and vice-versa. This is basic game design.
2. Charging a long bar just to kill one enemy is unfun. There should either be a non-charge attack for this or stragglers should flee in terror.
3. Shield enemies have WAY too much health. Shields should grant no more than 2 extra hits.
4. Powerup should not appear randomly for how much it affects gameplay.