Slitherine

I am hoping to do a Viking themed playthrough, and wondering which factions are the best at raiding?

Also, do any of the norse factions have access to two handed axe units?

Any faction that can raid and has the access to high Combat Power units and high value generals from the start. What you have to understand is that your entire stack combat power will be compared to Opponent Stack + invisible Garrison (governed by structures they have in the region and Military Expertise) + Fortifications regional value x 3. You wont lose much for losing your roll, but you wont gain much for the small wins either. Make sure your stack is at least 100 Combat Power against low level walls + garrison alone.

The real hidden in the Pillage action. Its available for sub50 Loyalty regions. One thing to know: Pillage occurs at the start of your turn before any movement takes place unlike Raids that executed after your stack moved. Regions have 1 cooldown turn before you can Pillage again, so you can rotate between 2 sub50 loyalty regions to constantly plunder them.