Well it seems you are way to new so a few tips:
-Mage does not win battles with his units, that's what spells are for
-Spells do 5% more damage for every point you invest in them and 15% more for every 7 point in them so with a lot of point you can easily do 2-4 times the damage
-Mage gets less leadership (ability yo control more units) the paladin or warriors so he needs to have few but useful and resistant.
-Gold is always a problem at the beginning but not that much on normal
-Try to get some defensive items for mage (+x to defense and some +% physical resistance)
-Try to not loose many units since the good ones come in limited numbers and is way to expensive to buy too many
-Recruit reviving units (inquisitor, paladin, get resurrection spell, etc) to reduce the number of units you lose.
-Use units that summon other units (royal gryphon, royal ents, druids, etc.) to suck up the damage to your REAL units.
-Look this page for some extra info (although is updated to AP):
http://www.celestialheavens.com/kingsbounty/hs-en.htm
-Try to collect all the runes, chest, etc. that is in a new area before fighting
-Try to get some +% fire/magic damage for mage since is greatly help
-Try to fight your first battles with similar level enemies
-Do all the missions to get runes for useful skills
-SPOILER: Do a kite-game (get all the island scrolls without fighting) so you can get to new lands, get money and useful items/spells/units and THEN start the game.
Have fun