For anyone still following the thread - or anyone new that wanted to check out the topic....
Over the weekend, I finally got my 500 Counselor trophy grinding in Private Matches - so I can confirm Private Matches work! I recommend keeping a tally from the get-go (or at least keep a tally once you hit 500) so you're not playing a guessing game the entire time.
My F13 partner and I always play Private Match/Random Map/Random Jason. Surprisingly, I still haven't unlocked the 500 Jason matches yet - or the 666 kills.
Since I got 500 Counselor, I reckon I must be close to 500 Jason. However, I've played oodles of matches against Bots (7-8 kills/game) and still haven't unlocked 666 kills. Kinda surprised it hasn't popped yet.
I'm beginning to think Bot Matches are a waste of time grinding toward the 666/1313 kills trophies.