The online here is pretty dead now, So unless you want to spend a year and a day even finding this many matches, then even longer finding them against people who aren't die hard KOF fans, well, the only way is boosting. So i'm going to explain the method Uuuuuwweeaaaah and myself are using to get to 300 matches, and naturally this also covers the 100 wins achievement too.
Firstly have the person who's going to be winning set up a profile team with Joe in it. The person who is going to be losing should set up a profile team with Daimon, Raiden and Maxima in (i'm sure other big characters work for this too, but these are the ones we've been using).
Next one of you hosts a player match and invites the other person. This way neither of you ever leave the lobby between games. Annoyingly, there's no way that i've found to do 1vs1 online, so if anyone knows if its possible, please let me know, as it would essentially cut the grinding time but about 60%.
Obviously, on the character select screen, pick your profile teams and the person who's winning should make sure that Joe is their first character. That said, if you have him set as the first person in your profile team, all you have to do when selecting order is to press start. Same for the loser, their order doesn't matter.
Right, when the battle starts, the Joe user should do his qcb + hk attack (backwards hadouken motion and b). The beauty of this move is that it doesn't cause a hard knockdown, it never misses and pushes the enemy into the corner automatically, and it does a nice chunk of damage. Due to the 3 people you're beating on being very large, they seem to take an extra hit per kick than normal characters do, making this method even quicker. Also don't forget to use your meter to shave even more time off. You gain enough to do 4 EX versions of the kick which do 20% of their health bar each. Also you don't even have to look at the screen while you just do the same motion over and over again at the same pace, which means you can distract yourself by watching tv or with other things.
Another mini time saver, which does mount up after a while; after you win the round as soon as it says "1P wins" or "2P wins" press a to skip to the next fight. If you leave it be and do it automatically, it delays by another 10-15 seconds.
I recommend switching who wins every 50 battles or so. Sure that's nearly 2 hours each, but that means there's less messing about on the options menu switching profile teams and believe me, if you're the one not having to kill 3 characters every fight, you appreciate it much more. Alternating wins just makes it a pain for both of you all of the time, instead of being able to have a break for a bit.
If anyone has anything to add, or any revisions they want to make, please leave a comment. A thumbs up would be appreciated, and if you leave a thumbs down please comment as to why so I can make the necessary amendments.