Cheats found by Daniel Marschall by manual scanning of the working RAM.
01F039D0: You are invulnerable!
0100AAD0: Your enemies gets killed when you beat them once*!
010950D0: Infinite missiles for Hou-Oh and Bakuryuu2 ("dragon")
010951D0: Infinite missiles for Juu-Oh ("rhino")
010952D0: Infinite flashes for Raijin-Oh
010953D0: Infinite shields for Raijin-Oh and God Raijin
010954D0: Infinite bazooka for God Raijin
010955D0: Infinite bazooka for Bakuryuu1
010956D0: Infinite gravity waves for Raijin-Oh and God Raijin
010104D1: Start and stay at round 1 (Enemy: Kabirian / カビリアン)
010204D1: Start and stay at round 2 (Enemy: Deadbear / デッドべア)
010304D1: Start and stay at round 3 (Enemy: Aburaa / アブラー)
010404D1: Start and stay at round 4 (Enemy: Marshmal / マシュマローフ)
010504D1: Start and stay at round 5 (Enemy: Skelton / スケルトーン)
010604D1: Start and stay at round 6 (Enemy: Biolon / バイオロン)
010704D1: Start and stay at round 7 (Enemy: Jrksatan / ジャークサタン)
010804D1: Start and stay at round 8 (Enemy: S.Jark / S.ジャーク)
* Please remember! In this game, you can only kill the enemy if you are in the "combined" form, that means Raijin-Oh for round 1 to 5 and God Raijin for round 6 to 8. Otherwise, the enemy will stuck at the life value "30" and will not get any further damage from you.
Even if the game was reviewed as "the baddest", it was one of the games that fascinated me in my childhood! And I do not know how I could play it to the end since there were no continues!
I also made an assembler patch that implemented a continue functionality. The continue hack which I developed can be found here. Have fun!