You don't have to, ur rune mages are super weak coz u dont have spare runes in ur inventory. Replace with another unit that summon units, or split royal thorns in half.

like zjad already said a QUESTIONABLE army selection:P

Is it good to kill elves on Arlania before i free Rezo from the undead?
Because after that i cant attack them, and some of undead from the beach desapear

maybe its better to clean islad first and then rescue Rezo?

Do everything you can do before Rezo, yes. This is probably the hardest fight in the game so you want to level up as much as possible.

i beat him with no loss, i'm lvl 43, but then i saw that i cant attack elves so i must to kill them first cause i want to rise lvl as high as possible

Yeah I'm slightly miffled that some random racial mobs will no longer fight you after being "friends". It is sort of nice, but now I am denied combat experience and a chance to raise my medals more.

Supposedly there is a spot where I can repeat a battle non-stop to get infinite xp. I think I saw the guy, but never triggered him.

Rezo's "attacker" (the undead knight?) was somewhat difficult but I used the overpowered rune mages so I won it somewhat easily after I hit a certain break point.

Mine didn't have 9000 skels though!
737 ghost, 379 vampires, 775 necromancers (this was the hard part), 4264 zombies, 2538 skels.