RTF is a global modification that replaces all towns in the game.

I can suggest levels 1 and 4.

I think level 1 could be: Patrol, Scout, Bodyguard
With level 4, the situation is: Warlock, Magma Warlock, Ragnaros' Servant

Added after 51 seconds
Or Ash Sorcerer

Added after 1 minute
Or you could take names from World of Warcraft, specifically Dark Iron Dwarves, and modify them slightly

Added after 2 minutes
And a level 2 unit with a helmet could be called "Disposer".