I have been leveling up Walker using the single player maps. I pick one or two and do them repeatedly. I look for one that is easy enough for only the first four groups, does not require manual troop control and enough buildings for the Leader to gain some experience. You want quick but a little chunky. Bonus for least troops used. MAKE SURE TO USE YOUR LEADER! ha.
My Leader has gained 3 levels, 15-18, in about 2-3 hours, game time. It is a great filler for when immune or between troop training. I'll have a full army cooking, when I get to about 20-50% made, i will start attacking the single player. The attack for me, will cost me 2-3 troops at the most, often none. Just add to the working queue each time you return and make sure your losses are not high. When full army is ready, attack PvP for glory. Rinse and repeat.
I have been using F.O. Shore. I drop the first groups in the corner, walk away to play with my cat. Next time I get up to refill my coffee.... I do it while doing other things. Set and forget, rinse and repeat. The important part is to find a map that is quick, requires minimal troops and attention, and is chunky enough(has enough buildings) to gain some exp for your Leader. DONT FORGET TO DEPLOY THE LEADER. Find a base that works for you.
-kmb08753, Death Pirates