How is experience distributed between my party members?

Solution 1:

From my experience, if a character damages a monster, he will gain full amount of experience when that monster is killed (e.g. 90XP for skeleton), but if he is not involved in killing monster, he will gain half of experience (e.g. 45XP for skeleton). Dead characters gain no experience.

For monster groups, each monster kill is counted as killing single monster.

And that is probably what makes your party having different XP:

• There are more groups in lower levels.

• Since most of (or even all) spells affect all monsters in group, your mage started gaining full XP from all monsters.

• When fighting group of monsters, fighters will first hit monsters in front of them (left fighter will hit left monster) and then hit monsters at the other side (left fighter will hit right monster if left monster is dead) and when first row is finished they will get to the back row.

• I presume you are using rogue for ranged attack, and in that case he will only hit monsters in front of him (if rogue is to the left, he will, in most cases, hit only left monsters, and when they die he will just miss right side monsters).

Edit: And as HenryHey said, they all get XP for finding secrets.

Solution 2:

From the Grimrock Wiki:

They can be obtained in various ways, primarily based on which class the prisoner belongs to:

• Fighters gain xp primarily by damaging opponents with melee attacks
• Rogues gain xp primarily by damaging opponents with ranged attacks
• Mages gain xp primarily by casting spells and using magic items
• The whole party gains xp by solving puzzles & finding secrets.

Solution 3:

To fix that, you have to use the Spirit Mirror Pendant, which increases the rate at which the wearer gains experience. When some of your team members level up slow, tell them to equip the necklaces.