I replaced my gas tank shield last year because it rusted in half. It was a PITA because the tank and shield are held on by the same bolts, so I had to change out two bolts at a time.
I don't think the shield is all that important but it does give the tank a bit of protection. Mine rusted in half right above the exhaust and I was worried about too much heat directly on the tank.
If I were you, I'd go at it with a tin snips and just cut away enough to get to the leak.
Our gas tanks appear to be coated in plastic except for the top and seams. I'm guessing yours is probably leaking at a seam where the rust creeped through into the tank.
I managed to use a soldering gun on a leaking tank and it worked great,... don't use any open flame though (I tried it and set my tank on fire).
There is a special glue called "Seal All" that even lists sealing gas tanks on the label, but solder is the best.