Suggestion: Allow crusaders with abilities that affect random enemies that are therefore shot-like, although not actual shots in Reign of Fire; additionally perhaps crusaders that cause random explosions/add extra spawns that explode. (It's just the most limited challenge apart from the T1 only one so expanding on the theme a bit)
List 1 (Shot-like): Gloria, Priestess of Time, Imani, Garnet
List 2 (Random Explosions): Fright-o-Tron, Skeletom, Holddoor, Gwen (Already in because clicker but still on theme).
It's not something we're currently planning on adding, but I'll add your vote to the item on the suggestion list and we'll see what happens.