You should be fine with those levels, though if you have a lot of trouble with the battles, go to some place like the Castle of Ordeals (fighting Zombie Dragons), Ice Cave (fighting the Eye) or the Sky Castle until your levels are 30-35+. Make sure your White Wizard has the spell Exit. You should also have 99 Heal Potions and Antidotes. Equip your Knight with Excalibur and Thief with Katana (I think you have it by now). Since this is FF Origins, make sure you've saved lots of Heal Staves and Heal Helmets, and Defence Sword and White Shirt are also useful.

Try going through the (SPOILER)Temple of Fiends Past. If your party is in a bad condition, wait until you get into an easy battle, kill all but one enemy, then have someone use a White Shirt and the rest use Heal Staves/Helmets. On the next turn, have someone use the Defence Sword and the rest use Heal Staves/Helmets again. Repeat the pattern but have someone else use the Defence Sword on the next turn. You can also try confusing the last enemy with a Wizard Staff. When you've got yourself healed, just kill the last enemy. Then continue.

The last dungeon will also let you gain a lot of levels. You can keep advancing and fighting there until you're running out of spells and items, then have White Wizard cast Exit, go to Coneria to heal yourself and restock your items, then enter the last dungeon again. You'll eventually be strong enough to beat the place.

One more tip: when you find the (SPOILER)Masamune, cast Exit and save the game. Sure, you have to go through the (SPOILER)Temple of Fiends Past again and fight the 4 reborn Fiends again, but believe me, it's a lot easier with that sword. Your Knight is already strong enough with Excalibur, so give (SPOILER)Masamune to someone like your Ninja, or even better, your White Wizard. The sword is the strongest in the game, anyone can equip it and anyone wielding it will get tons of attack power.