There are several games I used to destroy as a kid for Gameboy.
Navy Seals: Yes, I know the game is terrible. It's made by Ocean with the "plum plum, plum plum" music. For some reason, I just practiced at the game and became really good at it. The trick is to strategically use the Sniper attack at certain areas to destroy all the enemies and memorize where the falling pitfalls and ceilings are. Also, this game has 1 hit deaths. Yeah, can't beat this one anymore.
Wizards and Warriors X: Fortress of Fear. It took a long time but, I could usually beat this one. Not dying with the Boots of Jumping, memorizing enemy patterns, and just taking it slow. The 2nd level boss, if you can even call it that, is literally jumping into a skull, which acts as the exit for the level. Died so many times here. The other thing that makes it difficult is that there are ZERO invincibility frames, so enemies can just keep damaging you. Definitely recommend a playthrough, level 4's music is epic, BTW.
Looney Tunes. Really great title by Sunsoft, not many people have heard of this one. There is also a GBC version, same game, otherwise, just in color. Used to be able to destroy this game every time as a kid, not so much now. You only have 3 hearts of life, basically after 2 hits, you're dead. Used to love playing this one as you play as multiple antagonists (Daffy Duck, Speedy Gonzales, Porky Pig, Tweety Bird, Taz, and Bugs Bunny) Porky Pig's level is my favorite. It's a horizontal shmup and really fun.
Yeah, Skate or Die: Bad 'n Rad is really hard. You can definitely tell it was made by the same people that did Castlevania The Adventure. What throws me off is they expect you to fight melee bosses while you roll around on a skateboard. Turning around while doing this is very awkward and forget dodging projectiles. Can't even beat the first level.