Ashura

[Rengori]

Any tips on beating him/her?

Here are my levels:
Cecil: 39
Edge: 36
Rydia: 35
Rosa: 35

You'll notice I haven't been to the moon yet to get FuSoYa. That's because I die before getting to my destination.

I'll probably need help with Leviathan too.

[NeoCracker]

Cast Wall on her so when she heals it will bounce of here and heal you instead.

[rincewind]

levelling up a bit may also help, you'll then have the HP to deal with any attacks:)

[Crossblades]

The fight would've been easier if you had Kain

[SinSpawnAmadeus]

Casting Wall/Reflect=Win. You're levels are fine, there is no need to level up further. Don't bother getting FuSoYa, as you are unable to use him in the underworld anyway. Ideally, you should have done this BEFORE you lost Kain, but ultimately as long as you have decent healing spells, bolt 2 and blitz you should be able to take out both Ashura and Leviathan no trouble.

Oh, and getting the Excalibur wouldn’t hurt either. Look up a play guide for how to get that.

[Rengori]

Casting Wall/Reflect=Win. You're levels are fine, there is no need to level up further. Don't bother getting FuSoYa, as you are unable to use him in the underworld anyway. Ideally, you should have done this BEFORE you lost Kain, but ultimately as long as you have decent healing spells, bolt 2 and blitz you should be able to take out both Ashura and Leviathan no trouble.
There Is no Wall/Reflect in FFIV Advance. The spell names confuse me. And crap, I did NOT know I could go here before losing Kain.
EDIT: Nevermind, I needed to level up Rosa to learn Reflect and beat her quickly.
EDIT 2: Kicked Leviathan's Ass.