I believe it's a mating run fic where Stiles is an omega and Derek is an alpha, but it may just be an ABO dynamic fic cause I've tried every fic tagged/recced mating run everywhere. In it Stiles is forced into some compound where he doesn't want to be and he's there for awhile preparing for his mating. Theres one scene where he's in a yard that's separated by a fence and theres alphas on the other side of the fence "showing off". Then finally the mating run happens and Stiles is determined to not be caught. He finds Derek and Derek agrees to help him fight off other alphas that are chasing for him without going for him himself (but he's lying). In the end it's revealed the whole thing was staged because Derek is Stiles mate and he wanted to convince him to trust him.
It's not this one: http://archiveofourown.org/works/44