A friend of mine owns "Convicts". These fish are about 2 inches long are black with white stripes and are quite vicious. He feeds then with "feeder guppies". Throw one in and watch the poor guppy try to escape while several Convicts try to tear it to pieces. Lots of fun.
Now instead of having
1 guppy against several convicts, I propose having a tank that is partitioned into a "guppy" section and a "convict" section. The two sides would be separated by 2 screens (that pass water through but not the fish). The screens would run through the middle of the tank, but also be on rails on the top so that you could adjust where they are, you could also pull them apart to create a third section that has no fish at all right in the middle. This is the "arena" section.
The screens also come equiped with trap doors that can be pulled open quickly as to let out one fish at a time. When the fight is to begin, the screens are separated apart to form the "arena", and then one or more guppies are released and the screen is then closed (to protect the rest of them). Finally, you get to chose how many convicts to release, and watch the carnage!
Perhaps there is a way you could stop the fight if a guppy successfully stays alive for a period of time, but I don't know how that could be done without using a net to try to catch the convict before it eats the guppy.