For fireballs, move back and forth (from left to right) several times,
then press Jump. You'll take a hit, but the enemies will suffer greater
For limited invulnerability, hold the Up key for 2 seconds then press
For short-term protection, move back and forth (from right to left)
several times, then press Jump.
Super Plasma Bolt:
Hold the Up key for 2 seconds untill you hear "Charge up", then press
Fire. This will cost you one hit but the results are well worth it.