PDF made a mod increasing the cost of poor units *quite* a lot (999 resources actually), to prevent the AI from building some of the weakest troops. The thread in question is available
here .
There are also special maps that may fit the bill; for example, Orania Nasty Edition or Gandalf's random maps (Poke in the Eye and Random Opponents). Or perhaps even thrown in a few alliances between the AIs for added fun. Or all of the above.