You can use Steam to make this a little quicker if you own all of the clans.
1. Go into Northgard's options and uncheck most or all of the DLCs.
2. Remove your Northgard install.
3. Download the game again.
4. Now beat Bifrost with the clans available to play.
Once you beat Bifrost with the clans you have checked (maybe just the starter clans) the achievement will pop.
This saves you from having to beat Bifrost with literally every single clan available if you've purchased them all.
Afterwards just recheck your DLCs and redownload the game again to get access back to your purchases.