you got tents for unlimited skeletons, towers for unlimited mages, bonepiles for unlimited ghosts and treehouses for unlimited elfs.
You can have unlimited faeries with lv 4 druids surrounded by trees, water or rocks. They summon fairies to aid their side. The only thing that makes this method annoying is that lv 4 druids are too strong (especially if you begin the level with low lv units). And now the slime method: put some slimes (the higher the lv the more they split and the more they become a threat) in a place surrounded by in-accessible terrain (like druids) make a teleporter there that leads to the other one outside this cage, but make a bit in-accessible terrain around the teleporter so that only small and mediums (or only small ones) can get in the teleporter. I tried it myself with the scenario editor and it quite worked (the second method with slimes). Well thats it and sorry for my bad english, i aint english...
Daniel Paul Hightower
You still face the hard-coded 150 character limit in the game. Once the number of characters passes 150 on a scenario, the game will no longer allow slimes to split on their own. (Only way around that is for a player to split the slimes.)