I suppose if you could gather up every goblin on the continent and have them stand in one very big, tight-knit group and do nothing while you chain-cast high-level spells all day, you could waste a lot of time and energy and have nothing to show for it.

Said number of goblins wouldn't just stand still while you cast at them. They would charge you and slaughter you long before you could do enough damage to gain any XP. Remember that pre-3X casters would have their spells interrupted if they took damage while casting. 1 hp was enough to bring your most powerful spell to a grinding halt. I'm pretty sure a few tens of thousands of goblins all mobbing you at once would be able to keep you from casting anything but a shadow.