You soak the food in garlic oil. Frozen foods work quite well, you just unfreeze with a few drops on top. I think you cn also buy food with the garlic presoaked in. Ii've donethis, and with vitamin supplements. The theory partially goes that the fish tast so foul that parasites just let go.
I've heard the story that angels carry dieseases that affect discus, but I think it's an old wives tale, especially as you now see so few wildcaught angels. I've seeen wildcaught anything turn up with some ferocious dieseases.
I wouldn't put in the metrodizonal without some research - I would have thought an overdose will likely be fatal. It's the active ingredient in some drugs, but you're guessing at concentrations. I know people who deworm discus with cat dewormer.
My angels respond well to lots of water changes, like 20% every other day for 5 days.