If you're going to play Mage on AP with no losses then you're going to need to do some sneaking first! You also need to make sure you have the Ancient Amulet available somewhere as well as some other intellect boosting items.
To avoid wasted time and and disappointment,(especially on higher levels) i recommend you save at King Mark's castle at the start, then refuse the training in Debir and take a look at what you're going to get to start with. You might also like to grab the Bolo map and go and get the amulet so you can see what items will be available after you get the first stone. For example, in my current game i had the Gladiator Sword (10 might runes) available when i first arrived at Debir. After I got the amulet from the cave in Bolo and returned, the Ancient Amulet (10 magic runes) was available in the Debir castle, too! If you go to Scarlet, Bolo, Verona, Rusty Anchor, Tekron and then back to Debir, it should take less than 3 days and you should be level 14 or so by then. So you will have a ton of runes to play with and cash to buy the good items.
In terms of prioritising skills; Personally, playing as mage on impossible, i do upgrade the Might tree to level 2 training for the pet dragon because the Exp on imposssible is only 60%. This way by the time i get to Rusty Anchor, i have level 2 Dive and level 3 lightening available which effectively gives me 3 spells each round to play with. I also try and get the Mind tree up to Training level 2. However i guess on lower levels this may not be necessary.
IMO the Mage skills that are most important early on are Chaos, Higher Magic, Transmutation and Distortion. Having used a lot of might runes on the dragon, i would take Higher Magic level 1 before Chaos level 2 to start with. That way i can cast 1 buff/debuff and 1 damage spell for the first 2 rounds, which is enough to sail through Debir and most of Scarlet. Then as i level up i take Chaos 2 then Higher 2 then Chaos 3 and then probably Distortion 2 before Higher 3. Finally, i go for the extra 2 Destruction levels. I usually leave the Order until later as if you're not letting the enemy get at your units in the first place, resurrection is not so important.
Most of the boss fights are not so hard with a good combination of summoning units and spells. With some of the earlier boss fights though, sometimes it's helpful if you've found Dryads otherwise Beholders do nicely too! Split them into 2 or 3 seperate stacks along with maybe Paladins and Royal snakes or Griffins. First try to take out the mind immune units then keep the rest asleep while you blast away. Ideally you don't want to let the boss get a chance to cast! If the boss is going to get a turn, try to make sure you have one cast left so you can dispel any burning or poisoning effects. Later on you will definitely need the 50 mana Star spell that does Astral damage and Timeback, but you can get by with some good tactics (Think Chaos magic!) until at least Montero! Good luck!
P.S. I will try and post a good list of no-fight quests that can be done early game in this thread later.
EDIT: Done!
http://forum.1cpublishing.eu/showthread.php?t=11214