acid disintegration? That....shouldn't be THAT good at defensive barriers. Dissolving things is limited to surface area. If all his efforts are put into splitting apart the projectile immediately to dissolve it in time, then it wouldn't be that hard to flank him or just shoot something fast enough to punch through.
Assuming this stuff is aqua regia, a gold bullet would be fast enough to hit him before dissolving. Otherwise, glass shot or powder would at least stun him long enough that he can't block a second attack.