they would have to use an individual server to track all of the data if it was going to be done that way, instead individual mobs should be tethered to a count...
i.e. you kill a giant rat ... 1/10th of an amazing deed.... 9 rats later you are now going to get a message when you enter town...
or maybe you kill 7 rats... 7/10 of the way there... then you start killing gnolls... and maybe they are a 1/3 ratio, so you kill 7 giant rats and 3 gnolls and you would get something like
*Good job ______, on keeping those pesky rodents out of the way and slaying part of the gnoll population*
or maybe you kill 20 rats = 20/10 = 2 so you would get an amazing message or something...
kinda like a flag for each thing you kill, every time you leave town... but different mobs score you different messages and are of different importantce... like if you kill 11340958643096 giant rats, and 1 dragon then you get praised for the dragon and nothing about the rats is mentioned... etc. this system would be much easier, then they would have to make bot's for the guards... or script bots anyways, that are linked to the flagging server, and they just scan to see what you killed lately and whatever is of most importance they congratulate on it...
and by bots, i mean nice little computers that have somewhat AI....
www.santabot.com comes to mind. hes very intelligent for a computer you know...
just my thoughts on how to make a complex system a little bit easier o.o