On retry, it's easier to get the Gades Blade which if you have the undead ring equipped, kills everything in one hit:
Also, you can take the Gades Blade in with you to the ancient cave.

To get it you must defeat Gades the first time you see him (in gordovan tower) Here are some tips:

1. Get the Mind Ring from Parcetyle (equip it to Selan)
2. Get the Holy Wings from Parcetyle Castle (equip it to Tia)
3. Get the Franshiska from Parcelyte (equip it to Guy)
4. Get the Camu Armor from Tanbel's south-east tower (equip it to Guy)
5. Get the Kukri from Tanbel (equip it to Maxim)
6. Get everyone up to Level 40 or more before taking him on. See the section
     below.

Get the fire or neutral capsule monster up by feeding them heaps of eagle rocks (best place to get them is from the eagles outside tanbel)

 

How to get up to level 40:

Here's one way of many to do this:

A) I was up to Level 10 before I went into the Secret Skills Cave.
B) Maxim and Tia up to Level 20 before leaving Sundletan.
 

When you enter the lakeside cave, go through the first room, head left and
go through the door, fall down the crack in the floor, and collect experience here.
This is where I get up to level 30 in sundletan.

Be sure to fall through the correct crack in the floor above, go in and out 
the door killing the eagle each time. (They give you the most experience around
this area, and they give you eagle rocks to feed to your capsule monster,
and are easily killable with boomerangs.)

C) Maxim, Tia and Guy were up to Level 30 before Tanbel Tower.
D) At least level 40 by the parcetyle sword shrine (all characters)

Red Cores around parcetyle make collecting a lot easier.