I played a round of rumble pit and multiteam and this challenge was grayed out as "not currently available"  I noticed on multiteam it was active when I first jumped in, but when I checked again later it was grayed out.  Perhaps it only counts assists until you die the first time?  What the heck?

The challenge is broken, as soon as you die it counts as "failed". Neither Bungie, nor Halo Waypoint have posted anything suggesting that they are aware of the problem but the Bungie forums are crawling with reports of the glitch.

Don't worry the problem will be gone tomorrow.

There are a couple challenges that are broken like this one, hopefully they'll get around to fixing them.