Destroying the tank isn't going to be too difficult, however you must be very careful in order to avoid receiving major injuries (one successful shot may kill you or heavily wound your character). Either way, it's only going to take you about 30-60 seconds to destroy the tank and you'll have plenty of ammunition for the M18.
Reaching your target:
It doesn't really matter whether you've decided to make a new jump or go back to the small building. Try hiding inside this structure. You will have to be very patient here. Allow the tank to patrol this area and wait for a right moment to make a move. Try aiming at the rear, however you may also try hitting it from the side.
You will probably have to fire at least 3-4 times in order to destroy the tank. It all depends on where it was hit. It shouldn't be too hard. You only have to make sure that you're not seen. Destroying the tank will end this short objective, so you'll be able to proceed with other important tasks.