I seem to run into the same problem around the middle of the tech tree every game and it almost always ends with me quiting and starting over. The problem as the title would suggest is I hit a dead end with my trained troops and have no way to level them up and make them effective.
At the start of the game (assuming I'm not playing as a summoner) I usually build a full stack of spears lead by a champ and full stack of militia lead by my second champ. I have them hang about at either end of my lands, sat in cities and taking excursions out in to wilderness to clear monster camps and such. Occasionally one or two units will die, but they can be replaced. Despite the fact that the AI targets the noob units, the experiance of the rest can usually carry them until they can gain a few levels and hit points. As more troop slots unlock I start to specialise these armies a bit more with more varied unit types, making use of new toys as they arrive.
These two early stacks tend to kill all the lesser monsters (darklings, wolves, etc) leaving only the stronger beasts behind. By the time I get my third champion and feel I could do with building a third stack, there's nothing left for this stack/champion to do to gain levels. Taking units from my main stacks and giving them over seems undesirable, breaking the original stack's synergy, and (as the high level monsters can only be killed by the high level stacks) it only results in giving me two uncompetative stacks. In the end I bring my best to kill the unkillable and the problem compounds itself, leaving my third, fourth or even fifth champions solely in the roles of near useless -10% unrest admins.
So how do others get over this hump and continue to grow their armies?