What level should i be to*Spoliers*

[Red Mage]

beat Zeromos. Right now my party is in there mid 40.

[loaf]

That's a decent level. I beat that final boss at Late 40's. It's nice to be in the mid to late 50's though. If you have the right items though he can be beat at 40's easy.

[Sasquatch]

It slightly depends on the version you're playing, as well. I've finished in the late 30's, but usually it's late-40's or higher. Depends on your equipment and tactics. Simple things ... like, since you probably won't be attacking with Rosa or Rydia, equip them with a staff and rod, respectively, instead of a bow/arrows and whip. A good staff will increase Rosa's Spirit, and a good rod will increase Rydia's Intelligence -- since that's what they'll need to cast spells more effectively.

[Red Mage]

Well my team is

Cecil
Rosa
Edge
Porom
Kain

[Mindu]

He must have been toned down in the GBA and PSP versions, because in the PS1 version I had problems with him when my party was over level 60.

There is some armour for Rydia in the final dungeon that increases her magic power to the point that Bahamut does 9999, in addition to the rod that was previously mentioned. Rosa should be equipped with the armour that increases the power of her cure spells even if it means sacrificing some defence. A less known trick to making the fight easier is to turn down the battle speed; that way Zeromus will never be able to cast two Big Bangs before Rosa can use Cure4/Curaja. If Rosa isn't at a high enough level to learn this spell then you're probably screwed. It might be beneficial to use a Bacchus on Cecil because he won't be doing much more than attacking anyway, and it will increase the power and frequency of his attacks. Don't forget Edge can throw Excalibur for 7000+ damage and the Knife for 9999. If Rydia is knocked out it's sometimes better to leave her unconscious, seeing as Cure4/Curaja heals more HP if just used on four characters.

[Red Mage]

The batlle always seem to go the same way....

Rosa casts shell on everyone.
The rest of the party attacks Zeromus.
Zeromus does Big Bang.
Kills Rydia and Rosa.
Everyone else is near death.
Uses Phoenix Down on Rosa.
Rosa is getting ready to cast Cureja
Zeromus does Big Bang again and kills everyone.
Game Over.

[Mindu]

In that case you need more HP, and there's only one way you're going to get it: by grinding. I wouldn't recommend Shell whatever your level; Big Bang's damage is physical and Rosa should be casting Cure 4 or Life 2 every single turn. The damage from Big Bang can be reduced somewhat by having Edge steal Dark Matter from Zeromus before you use the crystal. It takes a few turns because the success rate isn't that good, but makes the difference of a few hundred HP if you have it. Anyway, if you're only around level 40 then you could probably reach level 50 in just an hour.

The grinding this game required was the only thing I disliked about it. Any other FF except II could be beaten without once making a conscious effort to level up.

[Sasquatch]

And if you want a good place to grind levels, one of the crystal-tile-floored rooms is full of Red Dragons and Blue Dragons. They're tough, but they give a hell of a lot of Experience and GP/Gil. Go to the Namingway cave on the Moon before you do that, and buy a stack of Ethers. (I got rid of every HP-restoring potion that didn't cure all of them, they're useless at this point, just go with Ethers.) That will keep Rosa stocked with enough MP to keep everybody's HP up. Then head to that section, keep running around and killing dragons (the Face enemy also gives good experience and money, but they're too much of a hassle to me), then Teleport out of there, buy more Ethers, buy more things for Edge to throw, rinse, repeat.

[Mindu]

It's too late for Red Mage, but I found the best place for grinding was Bahamut's Cave with just four characters. I leveled until Rydia knew Flare and Rosa knew Holy (around level 56), and went on to beat Bahamut without Fusoya. It took about four hours to gain ten levels. Kain was one level higher than Cecil when he rejoined the party; the party was around level 60 when I fought Zeromus, and it was still a struggle.