I am working on a Final Fantasy V GBA rom hack and have replaced 95% of all enemies just past Karnak castle (normal enemies and bosses). This modification, titled "Dimensions", takes monsters and bosses from different Final Fantasy games, and pits them against your team in Final Fantasy V. Some of them may be difficult; however, this isn't meant to be a difficulty (kaizo) boss hack. With that said, they will definitely give you a challenge!
If you're interested, message me in private so we can talk more over Discord.
4. King-Ochu & 4x Microchu
6. Guard Scorpion
7. Firemane & 3x Bombs