Right now, have well over 2,000 kills for Infrantry, Leader, and Specialist kills to move from Silver to Gold in my Campaign commendations....
What Campaign missions are the most populated with Covenant?
new alexandria has several banshee enemies that continue to respawn, if ur willing to take the time to just fly around u could do that, that or the pillar
my advice, check out the youtube videos titled Onyx commendations. I know it's a little cheating with checkpoints and whatnot, but it works and is the fastest way to complete your commendations in Campaign
I used the beginning of night fall for support role, infantry, leader, head shot, precision weapon, nice arm and infantry class ones. Got bored with it so never made it to onyx.