You need 9 humanoids for this. Monsters can't lift or carry humans in a tower. But they can be the final monster.
You will likely want to do this on an early stage. First get an enemy near your base. At that time send all your troops in your base.
Then bring out your first human character, and stand next to the enemy. Bring out your second human character and move to the same place (the character will climb on top of your first character). Do this 8 more times. The final character (the last you'll be allowed to bring out.) can be a monster, but needs to be on top of the stack.
Then targeting the stack (and actually the first player in the stack) have him attack. he'll be able to do one of a few tower attacks, if the enemy is weak enough it should be a simple case to kill him as all 10 characters will attack with melee damage (creating a combo as well)