The fishes must be compatable. Fast fish with other fast fish. Slow with slow. You get the picture.

I highly doubt that you are able to improve the situation even with the most drastic move ... which is ... remove all fish except the guppy, and allow the guppy to establish its terratory for a couple of days. Slowly add the most non-aggressive, slow fish one by one. By the time you add the most aggressive fish, it will be on the defensive, rather than offensive, as it has no more territory to acquire ... but after a few days, I suspect that the aggressive will bully the guppy again!!!

Just seperate them!!!