I plan to make several of these if there is any interest.

This one is more of an in depth guide and has a lot of my personal insights rather than raw data.

I will work in more info as I find the time but also in response to questions. I'm not sure many people are playing this. Please message or comment if you have specific questions and I'd be happy to try to answer.

There are 4 classes you can choose from. Here is a high level summary of those with more detailed info to follow:

Monsters:

One of several options at the start of the game. I believe there is a baby dragon, slime, and imp at start. These guys can change into a "random" monster by eating meat that is sometimes dropped by enemies. The process isn't random and detailed guides exist on Gamefaqs. They have innate abilities and cannot equip any items. Those abilities recharge when you rest at an inn so you wont spend much money on them. They do not gain stats by fighting. The only way to improve them is to "evolve" them into a different monster. Their end game potential is limited because of this. I didn't include a detailed monster guide because I don't use them.

Robots:

These guys do not gain stats by fighting either. They get stronger based on which gear you equip them with. Their attacks recharge when they rest at an inn which makes them great starting out, although any weapon you give them gets its number of uses cut in half to balance this out. They can be very OP early on but lose effectiveness mid to late game against bosses. They are weak against magic, hard to heal, and can only reach about half of the total Stat potential of Mutants or Humans. However they are great at pushing a single stat and doing massive damage.

Humans:

They level up by fighting. HP levels fast. Theoretically you could have 99 in all 4 stats. However Defense will not increase at a practical rate and it could take over 25,000 battles to max it out. Instead you rely on your armor for DEF and use them for offense and healing.

They do not gain any abilities and rely on weapons and spell books which have to be replaced. They can be expensive to use for this reason but also extremely versatile.

Mutants:

They level up by fighting. HP levels at half the rate of a human. Theoretically you could have 99 in all 4 stats. However strength will not increase at a practical rate and it could take over 5,000 battles to max it out. This isn't a very big deal as you don't need strength to be an effective fighter. What makes them in my opinion the best endgame class is that they can level defense. With max defense and armor they can achieve the highest defense of any class.

They gain abilities(up to 4) which refill when they rest at an inn and can use weapons and spell books. Early game these abilities are a great advantage but late game they can be a burden because they occupy a slot and cannot be removed.

The most effective groups will include a variety of classes. Running 4 of one type can be challenging and is often done for that reason.

4 Monsters: Incredibly difficult to get through the game. You will have to save scum a lot on boss fights to get the best outcome with meats to survive. Some people try this for the challenge.

4 Humans: Can easily beat the game with these guys but will have to grind levels. Their versatility allows them to use any weapon in the game. This is one of the easier groups to run if you're new.

4 Mutants: This group is a little more challenging than humans. Its still easy to win with them if you're willing to grind. The cannot use strength weapons but have massive defense potential and free magic.

4 Robots: Probably the most challenging option. You cannot heal effectively so to beat bosses you have to kill them fast using big damage attacks.

2 Human 2 Mutant: A great group. Everyone can heal and cast magic but the humans can use the strength weapons which are a little more powerful than agility weapons. 1 Person can use Excalibur which is a strength weapon to full effect.

2 Robots 2 Mutants: OP group. Mutants can heal and cast while robots can do a number of things. robots work best at dealing damage using strength/agility weapons and can make great use of Gungnir and Excalibur. They can also kill groups cheaply using things like SMG, tank and Samurai bow.

2 Robot 2 Humans: Much the same as above but without natural abilities and less defense. Defense is lower in humans and you don't need 4 strength weapon users so this isn't as OP.

Mixed groups: These can be the most fun. 1 robot is always good to toss in for massive damage using Gungnir which will otherwise be wasted. 1 human/robot to use Excalibur is alosgreat.

With this class you're pushing for the following:

80+ or 0 Strength
50+ Agility
0 natural Defence
75+ Mana

Strength is useful to use strength swords. These hit for up to 15 X Strength where the Agility weapons max out at 13 X Agility. The slight advantage makes a noticeable difference but the two attack types are roughly equal. The real draw here is using Xcalibur to mow down groups. At 0-70 Strength it hits for 1050 damage. For this reason there's no reason to level strength on this 1 Xcalibur user unless you plan to grind past 80 or use Giants gear. At 80 it hits for 1200. at 100 it hits for 1500. With giants gear and 99 strength it maxes out at just over 2000 per swing. It may be worthwhile to neglect strength in one human and get his other stats leveled before your find Excalibur. The strength Levels will pile up fast once he has this weapon in hand.

Agility is only needed for Humans to land attacks with the more powerful Strength weapons. It may also help them dodge attacks. 50 should be adequate to land most attacks but if you find yourself missing often just level it more.

Defense will not level. You have to use gear and can get 50-73 defense which is decent but will not fully protect you at the end game.

Mana can max faster than any class and help with crowd control.(Fire/Ice/thunder books mostly at 6400 GP each for 30 uses). These are cost effective mid to late game and in terms of damage can compete with weapons like tank(costs 36,000 GP for 20 uses)at a mana of 85 for a fraction of the cost. Flare books are devastating against groups late game but too expensive to use for grinding. At 85 mana they hit every enemy for 1100+ before damage reduction.

Healing effect is (User mana+ Caster mana)X4. This is partly why robots are hard to heal. With user and caster at 75 Mana each heal can do around 600HP. Having 1 caster spam heal rod every turn makes your group almost impossible to kill if they all have decent mana.

Mana also seems to reduce damage from magic attacks. Possibly at a rate of 5 X mana(unconfirmed).

With this class you're pushing for the following:

0 Strength
75+ Agility
75+ natural Defence
75+ Mana

Strength is not going to level efficiently on these guys. Unless you love grinding don't attempt it. There is a Trashcan Exploit that can be used to cheat this stat up but there's really no reason if you have a human or mutant on your team.

Agility is used for agility swords and can hit for 1000 damage at level 77 using catclaw. For comparison and strength user can deal 1000 damage at 67 Strength using sun sword and several others. It also supposedly makes you harder to hit, helps you hit, and potentially allows you to strike first. So its a significantly better stat to push than strength unless you're solely looking at damage output pr plan to use Excalibur(there's only 1).

Defense is what makes these guys god tier. If you were to level to Def 77 and use Parasuit you'd have a Def of 150. If my info is right this cuts 750 off of every physical attack that hits you(Assuming that damage reduction is Def X 5 which has not been confirmed). Either way its a great situation to be in.

Mana. As with humans, books can be very effective here(Fire/Ice/thunder books mostly at 6400 GP each for 30 uses). These are cost effective mid to late game and in terms of damage can compete with weapons like tank(costs 36,000 GP for 20 uses)at a mana of 85 for a fraction of the cost. Flare books are devastating against groups late game but too expensive to use for grinding. At 85 mana they hit every enemy for 1100+ before damage reduction.

But why use the books when Mutants have their own magic abilities? The books have a 10X modifier while those abilities are 5X( For Blizzard and Flame) or 7X for Fire/Ice/Thunder. This makes a much bigger difference than you would think because of how damage is calculated. I think its your damage - enemy Mana X 5. But again I haven't confirmed the exact calc. The abilities are great early game though because they are free.

Healing effect is (User mana+ Caster mana)X4. This is partly why robots are hard to heal. With user and caster at 75 Mana each heal can do around 600HP. Having 1 caster spam heal rod every turn makes your group almost impossible to kill if they all have decent mana.

Mana reduces incoming Magic damage. Possibly at a rate of 5 X mana but this exact math is unconfirmed.

Other abilities:

Teleport can be awesome up until Nasty Dungeon. After which you wont need teleport and can swap it out.

Heal is great for status effects. Late game it has less value as you wont be afflicted by these effects if you play your cards right(Use Aegis).

P-Blast(15 uses) and Flare(5 uses) are awesome if you can get them. God tier because you can spam tents at 10K a pop to recharge them in the dungeon. For comparison Flare book costs 50K for 10 uses.

Recover is in my opinion a wasted slot late game. See my healing comment above and you'll understand why getting up to 100 HP per turn isnt great. If you get it early game its a maybe but there are few opportunities to do so.

0-damage(Blocks thunder/ice/fire) and 0-weapon(halves physical damage). I prefer to use instead Dragon Chest(0-damage plus def boost) and Ninja Glove(0-weapon plus def boost) in these slots.

0-Change adds 0-weapon and protects from all Status effects. Could be helpful but at the end game when I am likely to acquire it I have no use for it. At that point Isis is using Aegis every turn and granting me this effect. Until the last boss who doesn't use status effects. As explained above Ninja glove has more value to me in in this slot.

0-All: Now this is an ability. It adds the effects of 0-weapon, 0-change, and 0-damage. I like the idea of it but honestly at the last dungeon I'd rather have the slot for an Ninja glove. Until the last boss Aegis will be granting you this effect. The last boss has no elemental damage or status effects. The 0-weapon part will help you but the Ninja glove has it plus extra Def.

Conclusion. They messed up in how you gain these abilities. If they had more of a chance of popping up earlier they'd be useful but their potential is mostly wasted end game.

Slot Shaming:

I recommend limiting your mutants to 2 abilities until the end game. At that time you can decide to add high level abilities as they come. To do this always keep 6 items in your inventory. They will not learn new abilities unless you make a mistake and run an item down to 0 uses.

As stated above my ultimate mutant doesn't need many abilities.

1 wasted slot for random abilities
P-blast or flare

If you happen to get an extra P-blast or flare maybe consider letting that take up a slot instead of a spellbook though.