Well before I used to get 1 Penal one Script run into two Grens and just keep merging the Penals. First fight I would usually end up with Vet 1 almost Vet 2.
I just messed around with a comp stomp and noticed one Penal squad (that took almost no damage) has 12 kills and still wasnt Vet 2. He ALMOST was vet 2. And he was killing mostly Grens. So that seems like an AWFUL lot of kills to not make Vet 2.
