Here's a basic breakdown of the Rule of Thumb system I built for balancing enemy encounters. Don't freak out at the mass of text here, it's actually a fairly simple process. I'll use your game as an example to help make things easier.
Start by setting the damage output of your player character at first level (in your case, Claude during his trials). In your preliminary stat build, use basic math. I mean double digit basic. As an example, let's place Claude's damage output at first level in the 3-5 range.
Frontline Fighter (Claude) Damage Output: [1.5] 3-5 [7]
The numbers in the boxes are for an expanded formula that includes damage alteration for critical strike [7] and botched attacks [1.5]. How you use the botch rule is up to you. But remember, don't overpower your player character, you want them to use skills, items, and spells wisely, and you won't be adding those numbers in until you've finished balancing the basic melee combat. Otherwise, the player will spam the attack button through every fight and become bored.
Next, let's set Claude's Hit Points for first level. If Claude were fighting himself, how many strikes would it take for True Claude to down Pseudo-Claude? I would say, for example, that it would take Claude 7 to 8 strikes unopposed to down himself. Therefore, let us multiply 8 strikes by the low-level damage output [3] to find his base Hit Points.
Frontline Fighter (Claude) Base HP: 24
Now, take a look at your first standard enemy type, which in your case would be "slime". When designing an enemy for an encounter, always think of them as another player, and build the fight from both sides. This will allow you to create more balanced, paced, and organic enemy encounters. Now, as a player, how many strikes would you expect it would take for you as a first level frontline fighter to take down a low-level slime in a one-on-one encounter? Personally, I would expect to take down a slime in two to three strikes. So, let's say three strikes from the low end of your damage output [3] will slay the slime, and two strikes from your upper damage output [5] will also slay the slime. Three strikes at low impact will total 9 points of damage, and two from the high end will total 10 points of damage.
Before we set the slime's hit points, here's another thing to consider: Critical strike. As a player, would you expect the slime to fall after a critical strike? I would say a critical strike would put the slime at critical health, say 1-2 points from death. So for the sake of example, lets set the slime at 8 HP. That way, it takes three weak strikes, two strong attacks, or a critical strike to hobble and any other attack that makes contact to down a slime in one-on-one at first level.
Enemy "Slime" Base HP: 8
Now, let's check things out from the slime's point of view. Again, in a one-on-one with Claude, how many uninterrupted strikes would you expect it to take in order to down the kid? Personally, I'd see my slimy self knocking Claude's head off after a minimum of say 8 blows and a maximum of about 12. So, let's take the value of Claude's HP we set earlier (24), and divide that by minimum acceptable strikes (8) and the result is 3. As this is a minimum of acceptable strikes, let's set three as the Slime's critical strike value. If we divide Claude's HP by the maximum number of strikes (12), the result is 2. So:
Enemy "Slime" Damage Output: [.7] 2-2.5 [3]
Now the basic encounter is taking shape. From here, you can give Claude one or two additional skills to aid him in combat, while giving the slime one skill with which to surprise Claude. Claude may have his Fencing skill as he does in the game, and perhaps a defensive or buff skill, such as the crest he carries around. In the Slime's case, why not give it the chance to inflict an ailment with it's standard attack. You could vary the ailment depending on the variation of Slime involved. For instance, a Green Slime could inflict a modest poison attack that bleeds 1-2 additional hit points over two to three rounds, while the Red Slime could inflict burning damage, and the Blue Slime could use cold to slow an attacker (Dex or Agility down) or even freeze them if the strike is critical (character is skipped for one round).
However, in regard to MP skill balancing, I would suggest that the players receive a point or two additional MP for every successful attack, and perhaps 4-5 additional MP for every enemy they down. And I'm using the term MP in regard to skills, not spells. Characters using skills should be find new vigor when an attack lands or an enemy falls, while a spell user should need to wait to build their magical charge or buff their mana level with an item. During the demo, I found that Salam's MP was fantastic, while the other two characters were struggling to put any kind of chain together.
From the base damage between Claude and the Slime, you could expand by creating base HP and damage levels from the other two characters, based on comparison with Claude. The same could go for finding values for the Bat and Zombie by comparing them with the Slime and then running encounters with the player characters.
When you're finished setting your preliminary stat build, you can then make it more dynamic and adjustable by multiplying every stat by a single value as a rule of thumb. For the sake of simple math, let's choose 10 (though you can also multiply by say 8, 12, or even 12.35 if you're feeling fancy). So, as an example:
Claude - HP: 240 - Attack Damage: [15] 30-50 [70] - Dual Strike [30-40 x2] - Family Crest [Negate Damage 10-20 Points]
Enemy "Blue Slime" - HP: 80 - Attack Damage: [7] 20-25 [30] - Cold [Damage Enemy Speed, Freeze on Critical]
From here, you can move forward by creating base values for your other player characters by comparing them to Claude. Don't be afraid to slightly imbalance player characters a bit with skills or spells, but make them comparable when it comes to base damage and HP. You can do the same for your enemies (like the other Slimes, the Bat, and the Zombie) by comparing them to the Blue Slime. Then feel free to playtest the encounters and tweak the expanded HP, Attack, MP, and skill values to your liking. Don't forget to give weak enemy characters a boost in stats or numbers when there are multiple player characters involved. Otherwise the player will easily overwhelm his opposition.
As a final note for this long-ass post, you can also tweak your enemy strengths and weaknesses based on the pacing of your game. I use the levels Prologue, Greenhorn, Apprentice, Journeyman, Master, and Legend. For Prologue I start off by making the enemies 6%-15% less powerful than normal; Greenhorn 0%-5% less powerful than normal; Apprentice 0%-5% more powerful; Journeyman 6%-12% more powerful; Master 13%-20% more powerful; and Legend 21%-50% more powerful, but I only use Legend for very important NPCs that I don't want the player fighting, but give them the freedom to engage anyway, or for secret bosses or engaging deities. I prefer to use alteration by Geography, meaning the wilderness around the starting town would be Greenhorn-level, while the plains beyond the wilderness would be Apprentice, the misty mountains on the trail to the Temple of Legends would be master, and the Monks within the temple would be Legend. You really really shouldn't fight the monks, but if you're going to be an rear end in a top hat about it, feel free x]
Of course, these are just some of my general Rules of Thumb. The most important thing you can do as a developer is playtest, tweak, and playtest some more. That's how you find the greatest balance.
