This is just temporary, put here because they might change it. Anyway, it's possible to train at 50+ monsters starting at level 20 (maybe even less) and tier 1 archers. What you have to do is fairly simple, you fight people on a regular tree map, you put your archers below the trees and lure the mobs to the right side of the tree patch in the middle and then run down between the tree patch and the seperate tree, push 'Standby' so your hero doesnt run back up. See example picture below.
All you have to do now is pay attention the archers dont start moving or the hero and you're set!!
To make it even more clear I made a lil picture of minimap, the Blue square is the frst place your hero should go to to lure the enemy units, the red square is the fighting area, between the 2 trees. The blue circle is where you should send your archars asap.