To log this CHALLENGE as a "find" and get the smilie, you must have found caches with 50 of the combinations of difficulty/terrain out of 81 possible, "Way to 81" (and find and sign this cache too, of course).
This can be found (and verified) in your geocaching profile - Statistics - Basic tab - scroll down to "Difficulty and Terrain of Caches I've Found" - count the number of colored diff/terr blocks with "find" numbers; or use the Project-GC checker link below (profile name and "Run checker").
You may find this cache and sign the log at anytime, but don't log as a find until you qualify.
All logs will be verified and logs not meeting the criteria will be deleted.
Difficulty is in the challenge and NOT the hide!
Congrats to B2Pi for FTF!!