The Welsh are my second favorite (after the Northumbrians) and I also blitz the Saxon dogs. I build as many troops as possible for the first 6 or 8 turns and then attack hard. I always attack more than one province at a time so they can't gang up on me. I eliminate the Saxons in about 6 more years. Then I work on taking the eastern rebel held provinces before the Vikings create too large of stacks there and then eventually work my way north. I can usually finish by about 860. I think the speed with which you need to attack actually helps your game over all because you get quite large quite fast and get in the habit of being aggressive.