It's weird that this has happened to both you and your friend and almost no one else. ;o is this PC, PAL or NTSC? Knowing which version it is won't help though, I'm just curious.

You seem to know that Tifa is the one who has to do the killing, so that's good. So here's a little check list for you:

1) Kills made with Odin's Steel Cutting Sword attack do not count.
2) Kills made by residual poison damage, i.e. the damage taken each round, not
the damage caused by the initial casting, do not count.
3) Kills made by enemies you have manipulated do not count.
4) Kills made with Death Sentence do not count.

unorthodox ways of killing, but worth mentioning I guess. and lastly, Tifa has to kill 96 enemies to get that next limit.

Did you get Somersault?