I'm far from a pro, but here's what I know:

1. How to properly spread out your units, since the rule of three doesn't apply here for sure?

With the right approach, you can acquire artifacts, mines, and resources on some maps just by using heroes. How? You pick them up/capture them, get attacked, and then run away to your castle. That's it! :D

How do you even play as this Barbarian? Without magic...

It's very simple. If you know how to :D For example, I play with all factions on standard maps and complete them on the highest difficulty level. And that's despite the fact that I don't play very well :D