The Challenge called “Combo Toss” which is to “Headshot and Enemy Spartan immediately after damaging them with a Grenade in PvP” is completely broken, and does not track or complete regardless of whether you are using a Shock Grenade, Plasma Grenade, or Fragmentation Grenade: I have not scored a certain headshot with a spike grenade yet, but I assume it’s broken too.
If anyone has had luck completing this challenge let me know.