How to complete the Smash Head badass challenge Stiff Competition in The Fridge?

This badass challenge takes place in The Fridge, and is Stiff Competition: Do not kill any Midget Boners before killing Smash Head.

So I've twice now killed Smash Head, leaving his three Midget Boners alive, and not been awarded the badass challenge. I clear the area before opening the loot chest which awakens Smash Head, so I only have to deal with him/them. I then retreat to safety in the entry area and snipe him from a distance. Sometimes he follows me into the entry area, so I have to retreat farther down the hall. Eventually, I whittle his health down and kill him. When he dies, the three Midget Boners are still alive, and come after me, so I know I didn't accidentally kill one of them.

I'm wondering: is the challenge only awarded the first time you encounter Smash Head during the quest to find the Goliath's lost loot crate?

EDIT:

Based on suggestions below, I tried the strategy of staying in the area and performing a close-up kill of Smash Head using a high power shotgun. Same result, the three Midget Boners are alive and still no badass challenge is awarded!. I think I've found a game glitch...

is the challenge only awarded the first time you encounter Smash Head during the quest to find the Goliath's lost loot crate?

No, you can get this challenge at a later date. I just accomplished this with my Commando by going back to my Normal Mode playthrough with level 41 weapons and quickly killed Smash-Head before his minions were able to come after me.

Although this question was posted several years ago, this might help another player to complete it. The player posting the original question above unknowingly states why he was not awarded the challenge: “I clear the area before opening the loot chest...”. Nowhere does the challenge state that there are only 3 midget boners. By clearing the area first, he had already killed some of them, thereby eliminating the chance of completing the challenge. You have to rush in and trigger the loot chest, kill Smash-Head, and then clear the area. You may need to run back inside the entryway to regen shields and health before taking Smash-Head out. I used the Fervid Gospel SMG to quickly dispatch him. Keep this same thought in mind for any challenge where you must destroy the boss without killing any of the secondary characters, including robots and flyers.