Death Valley battle walkthrough

The Death Valley battle is arguably one of the most challenging parts of the game to date.
If you are having troubles getting past this battle against the bandits, here is a guide to help you out.

1st off, always be prepared. This means crafting your gear and even some potions should you require them. You should surely upgrade Lust's armor and craft Hottie's armor as well as upgrading Demonica's claws if you intend to use her in these battles. Don't forget to equip these items.
Sleep before you head out! Make sure your fatigue is at 0.
Be sure to bring along Hottie, we will need her fire skills.
Equip your elemental stones, especially the Earth and Fire stones. For example, put the Earth stone on Lust and the Fire stone on Mina (or whichever 3rd character you wish to use).
Apply the attribute points on the 3rd character of your choice. Here you might ask, but Mina is automatically added to the party when we head out into the Wastelands. True, but if you lose the fight, you can then change her with someone else for your second attempt.
Equip the Demon Artifact on the 3rd character of your choice.

Now that everything is ready, head out and face your opponents. This fight will see 4 waves of enemies featuring 4 enemies each wave. In this scenario our party will have Lust, Hottie and Mina.
It is recommended to use the summoning skills Gaio and Core at the beginning of each wave. They both hit all 4 enemies, Gaio also gives some extra armor protection and Core applies a burning effect that will damage the opponents at the beginning of the round.
Feel free to use Hottie's flame attack, it works great against group of enemies, but be sure to leave 120 MP available for wave 3 and 4. So once 1 or 2 enemies are down in the 1st wave, the easiest wave, you might want to have her simply on guard/defense for the rest of the wave.
Lust should just punch away at the enemies and Mina should mainly use her Vampiric Drain to keep her HP up while doing damage. It is recommended to focus fire on a single opponent to bring it down as soon as possible. This will reduce the incoming damage.
Wave 2 the bandit leader will join the fight, he hits hard, so he should be your primary target. Bring him down fast.
Wave 3 will see you facing the bandits as undead creatures. The undead have specifc attributes. Hints to this where presented during the whole undead intro at the Sun Palace. There it is said that they are hard to bring down with conventional weapons, but then we see Hottie burning the undead to ash.
This translates into, they have a resistance to physical attacks and a weakness to fire. All physical attacks will do 50% less damage while all fire attacks will be doing double damage.
Here is where Hottie's fire skills as well as the Fire stone that summons Core become very effective. Beware, Mina's Vampiric Drain only works on living creatures so it will be completely ineffective against the undead.
So wave 3 and wave 4 you will mainly rely on the summoning abilities and Hottie's fire skills, especially her special Hyper Fireball for which we have saved the 120 MP and which will be utterly devastating.
Lastly, be careful with your fatigue, this guide will see you maxing out your fatigue, so take that into account and make any necessary adjustments.

Following this walkthrough you should be able to vanquish these foes. Best of luck ;)