There is some information here: viewtopic.php?f=25&t=43107&p=467724&hilit=metronidazole#p467724
You can also try injecting the medication into mealworms, fly larvae, bits of shrimp, gel foods etc. Or as a last resort, soak the food in the medication and feed it out.
There are several effective dose rates for this drug depending on what condition your are treating for. It comes in tablets and used to also come in a liquid suspension (which is also not very soluble in water). Oral is the best route and it can be challenging to find the body weight for a discus based on length alone so a gram scale would be useful (place a small container of water on the scales and then add the fish - subtract the weight/volume of the water to get the weight of the fish).