The way this works is that you will reset one of your choice Skylanders, with no upgrades or levels, nothing. Afterward, you must go through all the levels of the main story in NIGHTMARE MODE!
Alright, so this sounds like an old Nightmare Challenge. But the key thing here is that throughout the entire playthrough, you can only use Sheep Creep. This challenge will prove who are the best players, and who is willing to use the weak to win strongly!
Also unlike other challenges, I will give special credit to those who succeed this challenge, and for those who are competing in it. Each character has a special title if they complete this challenge, and so I will list them with your name when you are completing and have completed the challenge. You ready? Sheep Creep is! *baa*
Current Sheep Creepers:
Medicus with S3

StormDragon21 with

Wreckingball13 with

Badwolfmichael with

ZapNorris with

Zookinator (Me) with

skylandsfan with

Dark Snap Shot with

Yoshicrashcart8 with

ppmdoesskylands with

Eruptor100 with

drobot007 with

blade430 with

EnderBlazeMC with S1

Wall of Completed Sheep Creepers:
[None]
Oh, and one more thing, if your character goes above Level 5, it must be reset. Right, right.
UPDATE: Thanks to the concern of Sheep Creeper blade430, there has been a change in the rules. Players may feel free to upgrade and level up their Skylanders fully in their regular save files AFTER they defeat the Golden Queen. Keep in mind, however, that there might be another update involving this, as I haven't even started it yet.

UPDATE: Ok, so now that I have tried it out, new update. The level restriction is lifted, but you must level your Skylander of choice in Nightmare Mode. Also, 2 upgrades can be bought every time you defeat a Doom Raider, sans Gulper, as he is the first one and the easiest. Also, these upgrades can only be bought in Nightmare Mode as well.