This isn't really a solution, more some advice for people trying to unlock the various kill related achievements as quickly as possible - at the end of every boosting session you should play a quick one round local match against a second controller before quitting to dashboard or turning off your xbox. I've done this and unlocked Seriously yesterday pretty much bang on 10000 kills. I've also unlocked all the weapon kill achievements after 100 kills.
I think what's going on is that sometimes, for whatever reason, the progress you make in the last match of a session isn't added to the various achievement counters. I first noticed this when boosting the 'Win 20 matches on map X' achievements - if I won 20 matches in one session the achievement would pop, but if I spread them out across multiple sessions I'd have to play 21, 22, sometimes 23 matches depending on how many sessions it took.
Of course, it's possible I've just been really, really lucky - there's no way I can prove this works. If people start trying this and it doesn't work out let me know and I'll take this solution down.