you can't really "farm" a level 20 infinity. Doc Mercy (the one who drops it), is level 10(ish) correct? And because enemies wont scale in normal mode, he wont get to any higher of a level. Therefore he can only drop an Infinity around level 10. If you REALLY want to, you could find a chest and farm that for 6 1/2 days to get one.
also, whats with all the fascination with Infinity pistols? Even with never having to reload, I find them to be garbage. --- You guys can have your shorter kill times with your fancy sharp swords and bludgeoning weapons. I'll spend the extra 5 minutes looking awesome. - Typhlax