Magic the Gathering discard rules?

You can't make an opponent discard what they don't have. They would have to discard their entire hand, up to the number of Swamps you control. If the played an instant on top of the Sludge, that would be six cards discarded or whatever. If you cast a spell that has a definite amount attached to it, then the opponent having less of what that amount is would be your loss. If you wanted to not "waste" part of that spell, play something that has a X in the cost so you can control the effect better.