Start by putting your guppy in very brackish water. Columnaris can't survive in salt water. I think you want at least 1.003sg. I'm not sure the limit for guppies, but I know mollies can actually go in full marine salinity (1.024sg). I think this will take care of external columnaris infections, but probably not internal.
I've had my mollies in 1.010sg for the past 12 days. They finished a 6 day kanaplex treatment 3 days ago, and finished a 9 day oral nitrofurazone treatment today. I'm about to move them into a tank where I can better observe them (they've been in a 5g bucket). Some of these guys have had it come back twice, so I'm hoping the combination of salt, kanaplex, and nitrofurazone has finally licked it.
I haven't heard that Metronidazole helps with columnaris. I'd suggest you first try all following (those listed on the same line can be done in combination):
- kanamycin (kanaplex) + nitrofurazone
- maracyn + maracyn II (make sure your pH is below 7.5)
- triple sulfa
Also raise the salinity when treating. I'm guessing maracyn II probably isn't going to work in brackish water because of the pH. Maybe you can just try adding regular aquarium salt instead, since I don't think it will affect the pH.
Unless you catch columnaris early, it is likely also an internal infection, so try treating orally. My understanding is that kanamycin is the best (and maybe only) of the above antibiotics to be absorbed by the fish when doing tank treatments.
I read that pimafix might be affective. I'm not sure about using it in combination with any other meds.