This question has not been updated since the app update where we can train with six Pokemon, so what are the exact numbers for how much prestige is gained now? It used to be 500+ for defeating a Pokemon with less CP than yours, and now it's quite a bit less, in the ballpark of 200.
Some weird stuff can happen, like what if you mostly kill a Pokemon with a powerful one of yours, then quickly switch and finish it off with a very low CP monster? Would that get more prestige than killing it with the higher one?