Unity of the Elements

I played it quite a while ago, so I might have forgotten some things... Where can I get so much experience to unlock almost all the magic skills at the beginning of the map? It requires at least 25 levels, and even then, only the "necessary" skills will appear. (For this, you also need to learn a bunch of useless skills for this map, like catapults, mentorship, etc.) Or you need a ton of gold and a mentor. Maybe I played it like a noob and didn't notice or remember some details?