Ah, an age old question. There have been several solutions but the best one is the usage of 'killed' eventhandlers.
After doing some searching I came across this bit o' code:
{_x addeventhandler ["killed",{[_this select 1,player] exec "killcount.sqs"}]} forEach thislist ;
Put that in an east trigger covering the whole map.
_this select 1 = the unit killed
player is the unit who killed the guy with the eventhandler
killcount.sqs
_killer = _this select 0
_unit = _this select 1
?(!_killer == _unit):exit
kills = kills + 1
Init.sqs
kills = 0
Syntax not guaranteed. The solution I found used a different script entirely. There is a way to track all kills made, but I'm assuming you want to track the kills of only one unit.
To check when the global variable 'kills' reaches 10,20,30 etc, I think all you need to do is make a trigger that has:
?(kills == 30)
in the condition field, and then put whatever you want in the activation field.
As I said, none of this is guaranteed, but it looked to me like the best solution. Off the top of my head though I can't see why any of this can't work, but then I haven't had a chance to test any of it.
By the way, Mad Pup, I suggest you make use of the search feature. You'd be surprised how many of your questions can be answered this way.
Regards,
Gruntage