The challenge in "DROD: Gunthro and the Epic Blunder" room "Inauro Canyon: Once North Once East" wherein you have to clear the entire room's enemies without summoning any allies with a horn. It's a tough challenge but not as challenging as the sword-only challenge in Tuenan Armory. The challenge and achievement are officially titled "Impressing the Ladies".
The trick is to carefully examine the doors and pressure plates and strategically pick and choose which ones to open at which time so as not to flood poor Gunthro with roaches laid by the roach queen. It's also important to lure roaches back onto pressure plates to shut other doors for the same reason. Carefully lead roaches into narrow areas so you can do a two-column/two-row rapid swipe, and also keep in mind roaches can't occupy the same tile as a horn.
I'm still pretty new to DROD so there's a lot of unnecessary deaths here, and mistakes which look obvious on rewatch, but I still managed to pull it off with my own brain, which now hurts.