Giancamo; in CW, when you level up, you don't choose now, you win a set amount of leadership and a random number of runes accordingly to your class. You now have to invest in abilities to get more INT and choose items that give you it (or items/skills that enhance spells).
The order of creatures in battle is determined by the initiative. Units with more start before those with low in a global way (yours and you opponents). Look here for their stats and other useful info:
http://www.celestialheavens.com/kingsbounty/hs-en.htm (you can also look their stats in battle but not before them)
You have to choose items/skills/units ans spells that get you mana, transfer rage to mana or regenerate mana. There are quite a few of them. Since you are a mage is way harder to have rage to use the dragon effectively (a weakness of the devs since the dragon is way too useful for warriors but not to the mage).
To have a lot of mana get the Wisdom, Concentration, Meditation, Transmute and Thesis skill, Absolute balance, Scouting and Learning if you have excess runes.
Get the following items:
Portrait of the Queen of ice +20mana
Well of mana belt: +3 mana per turn /+20% mana regeneration
Archmage Staff: +20% lighting spell/unit attack /+5 INT (can even get 2 of them)
Perfect Shoes: +30mana
Magician cape: +20 mana /+30% magic resistance
Crystal helmet: +10mana
Dress of the mages: +4INT/+30% Archmage crit/+800lds
Necklace of firestorm: +30% fire spells
Skull of Death: +5INT/ATK
Diploma in anti-magic: +2DEF/INT + 20% magic resistance
While invincible enemies can look "invincible" they are not that impossible (although try to search for strong or lower if you don't want heavy losses). If you know an invincible enemy blocks the way to a castle (new units/items/etc.) fight it, the reward will help you greatly even with some loses.