I really didn't have much trouble with Zeromus, but this was probably due to the fact that:
1) I basically power-leveled before fighting him (all characters in high 50s, low 60s)
2) I went out of my way to get ever item I could before fighting him and
3) I only got as far as Zeromus in the GBA version; I played the PS 1 version before and lost interest upon getting Edge.

IN all honesty, the only final boss I've actually had trouble with was Sephiroth, and that was because it was the first FF I had ever played and I had no idea of the concept of "status ailments" and thus didn't bother to prepare to protect myself from them when he uses Pale Horse.

Really, I kind of acknowledge my survival against Zeromus to either be my power-leveling, or pure dumb luck.