In Gurps, how powerful do you need to fight a tank?

[Rupert]

Originally Posted by David Johnston2

It's the only option you've got with a super who is impervious to small arms fire apart from another super.

No, use an anti-tank gun, or a tank, from a nice long distance and concealment - it's far more likely to hit even a fast super (basically, you're using it as a very large sniper). Or, if they're only just immune to small arms, anti-aircraft cannons would be a good choice - more powerful than small arms, very high rate of fire, and set up to engage fast moving targets.

If they're immune to that they'll also be immune to field artillery, so at that point you've got a whole new level of problem. Maybe they'll come within 10-15 miles of the coast and you can hit them with a battleship shell, assuming you have a battleship. Otherwise, it's a matter of massive bombs and missiles or nukes or (more likely) finding a vulnerability (poison gas, catnip, whatever) and using that. Or, as you say, another super.