Fire Team Zulu was a Halo challenge site I started around ODST back in 2009-2010. It was centered around ODST's firefight, and later campaign, then started going to the other games. You can search the front page for mentions of the site before, and the original site is available here:

http://www.arcademy.net/legacy/fireteamzulu/

It was a bit like your mega battle challenges, except I wrote a light "story" for each challenge of completing a level or firefight session, and impose various restrictions on completing it, like no use of snipers, etc. Like this:

http://www.arcademy.net/legacy/fireteamzulu/?mode=viewgame&gameid=173
http://www.arcademy.net/legacy/fireteamzulu/?mode=viewgame&gameid=171

Originally you verified challenges by submitting links to games, and later usage of the stats API was planned. Then MS stopped the API completely, so I don't trust using any API that sources data from them.

Now you can submit a simple pic proof. Although the non-stat games are mostly honor system (They were on the original FTZ as well).

I need to add a couple of features. Linking to specific challenges again like I could on the old site is one, since right now I just have to send you to the central page. I'm also planning on bringing back the story text to give more context and make it a bit more fun to read them again.

Completing challenges gets you points towards the site's ranking system, which would be useful for other things, prizes will be available from time to time for a challenge, etc. It's been brought back now because I finally have a nice backend I wrote up myself to do updates - on the old FTZ, I had to wait until the actual day to enter a challenge into the site's database and then post about it. The new backend I can make content months in advance if need be - it all auto updates and such now :)