This is... the final floor!
Offscreen Voice from Waaay Back At The Beginning: This is my world... who dares step into it? Who?
Voice: I am complete darkness... I am Zeromus, but at the same time, I am not... I am Zeromus EG. It's been a while since I've seen humans... Don't fail to amuse me...
Okay, so there were three versions of Final Fantasy IV on the SNES/Super Famicom. There was the original version that got ported onto the GBA that you've more or less seen here, and a rather dumbed-down version that in Japanese was called "Easytype" (and retroactively, the other version was "Hardtype"). Characters lost abilities, items and spells were dummied out, and -- notably for this context -- Zeromus's battle sprite was changed to this centipede-cyborg-guy with an elf girl growing out of his crotch.
The game ported to the USA was neither Hardtype nor Easytype, although it was more similar to Easytype. But it had differences all its own, for example, Cecil had his Darkness attack in Easytype but didn't in the USA version.
Anyway, this guy is Zeromus EG, "EG" being pronounced about the same as "easy" in Japanese. They couldn't just call him "Zeromus Easy", though, because... he's harder. What did you expect, he's a Final Fantasy postgame superboss!
Zeromus EG opens with Big Bang, then follows with a combination of Toad and Mini.
He combos Flame, Lightning, and Earthquake attacks and can hit four or five times a turn. That's enough to overkill Brent or Niku, if he concentrates multiple attacks on one target.
He has counters, too. Early in the fight he casts Reflect on himself if you hit him with magic, so it's kind of a good thing I didn't bring Zara. Later he casts Absorb to restore his own HP, but that means he's on his last legs. He starts hitting a little harder and countering attacks with Curse, but it just takes a little more damage to...
Send him slowly offscreen.
Zeromus EG: Now is the time to take a short sleep... I must rest... Farewell...
Your reward for beating Zeromus EG is... that he gets to go take a little nap. No, really. That's it.