For some reason, my warden kills haven’t been counting even though I get the 4000 points and the kill. Could someone help me with this because this is preventing from getting the forunner destroyer. I have killed about 20 wardens in firefight, but somehow I only have 4 out of 10. Before you say no they counted, the 20 I killed were in between 3 and 4. This was all before I can get the warden pack. Could someone help me figure out why
Unfortunately the Warden kills only count if you get the last shot. 
Yeah you probably just did not get him, I can recommend tips and tricks for getting the last shot? Start a different thread in the appropriate forum and we can discuss it.
I really depends if you have the last shot on him. If you and 4 team mates have caused multiple damage to Warden, you will all get the kill (for a mythic boss).
But if you want the actual kill for the Warden, you would have to either place the last shot that killed him, or assassinate him at the last minute.
It’s because you didn’t do damage when he died