You should only have 1ish enemies up north, Try to confed him and stay with one region. Move to Britannia and take them out as quickly as you can. Around turn 100ish head back to norsca and confed them. Every region should now be built up. Finish Britannia and then move to empire.

I personally went through with that run like a week or two ago, just before Dwarves. The big key is that, at least if you have Wulfrik, you really don't need to care if they're taking some settlements. However, that being said I DID in fact hold my settlements, it's just mostly about confederating Norsca by around turn 15-20, Throgg being the bugger there since he probably confederated another tribe or two. Luckily, he'll still pretty much serve himself up right to you if you're in the area.

As for various groups among the Empire, sometimes they'll leave you alone. You might even consider looking at them to find if there's one or two who might go with a non-aggression pact, since that helps them look the other way a bit. If they're going aggro, though, you're gonna want to try to force them to come back home by sheer aggression. I ended up sacking two or three cities while they got my pittance of a village back home, leaving Wulfrik in a very comfy position from all the gold.

Throgg (or whatever you're second army leader is) will be nearby the settlements, maybe picking on them here or there, but try to find the weakest of the bunch for raiding. Another thing I did was virtually ignore the horsemen lineup, though that's probably personal prejudice. The literal only chariot I go for is the Ice Wolves Chariot. I also ended up making Wulfrik get to Lightning Strike asap and then moved into just cranking his personal power to eleven instead of focusing his redline skills until much later into the game. That lets him devour in melee, and I didn't even put him on a mount until his elephant was available.

Waaaay later in the game I even got ahold of the Sword of Khaine, and let me tell you was that just nuts!

Hope that helps! I may have digressed somewhat, so let me know if that didn't really help at all and I'll try to fix that. ^_^

(Btw, I think Norsca is getting their update here soon along with the Tomb Kings, but I don't know if that'll be a bit, so feel free to play them now AND later! :D )