Really..the best way to level early is to get a light crossbow, two quills of +3 bolts, a decent bladed lance and go solo sea raiders. Also you'll need a Heavy Courser for speed.
Just run circles around the raiders until they all run out of ammo, then shoot them in the head with your crossbow.. moving each time you reload. Aim towards the bottom of their shield and you should hit them in the legs.
135 xp a kill. You'll level up in no time
Have all your troops hold position until you get to lvl 20 or so. I don't even start recruiting until I hit lvl 20, with 6 training skill.
You can start with looters until you reach lvl 3, but really, Sea raiders are just as easy to kill and give more exp.
Rhodock and Swadian footmen are good as well. I try to avoid archers, but it really isn't feasible. Archers are defeated by the speed of your heavy courser and your own strategy. Eventually they run out of arrows and it becomes a duck shoot. Sharpshooters can be a nightmare while soloing. Avoid them at all costs.
Try to avoid hills and trees. Khergits are easy if you exploit the barrier.