Normal no-loss mage is completely plausible. You will need to employ some "strange" tactics.
Try to do a lot of map stealing and do quests that do not involve fighting. That will get you well within level 18-20 if you do it properly and get all the way to Reha. You may lose a week, but it is worth it. You will most certainly end up with a bunch of dragon eggs, tirex eggs, a lot of paladins, lizards, etc., as well as some decent spells, 200k+ gold (that's on impossible, so on normal it will be at least 350k), cool artifacts and so on. You do need to be careful, to save often and be nimble
As for the dragon eggs that hatched chicken, that is a prank. however those chicken lay golden eggs. They will not lay them for you, but they are sold for 2500 in the castles. The Dwarf on Tekron has a special line in the dialogue with him, with amelie asking why the dragon eggs are so cheap. Try it out. The normal dragon eggs cost over 10k. Those were only 2k. You should expect some foul play here
As for the spells it is completely up to a personal preference. In the beginning, your mage will not be that effective. Its true firepower shows around lvl 35+, where you will be able to upgrade some of the items and stock up on intellect. If you expect to do some damage with 10-15 int, you won't be able to, so I'd rather use debuffs at that time and an occasional 2x flaming arrow to boost the fire mage medal and grant myself some extra mana.
EDIT: As for the leadership doesn't matter, I totally support that it doesn't. Provided you have good tactical thinking and know your spells and army well, you can win a battle with an enemy that outnumbers you threefold without losses