Ooh, fun question. I'm having trouble even picturing 20,000 mice, but quick Googling lists the weight of a bear as 300 - 780 kg and the weight of a mouse as 16 - 27 grams, so to compare against a bear in terms of weight the mice would need numbers between 13,000 and 50,000. Frankly, even if this worked though, the number of casualties involved in such a fight would be such that I'd be rather disgusted with the mentality of the party who considered it a good idea. Maybe if they didn't know it was coming and that was their only option it might be reasonable, but that would require you to be writing your campaign using the Final Fantasy IX school of plot.
Assuming they can plan however, some kind of trap would be the approach I'd expect almost any party to take. Given the small stature of a mouse, their best bet would likely be to use nature and their environment against their enemy. The most obvious things that spring to mind would be knocking it off a cliff, dropping something heavy on it, or causing something else (a poisonous snake seems the most obvious choice) to attack it for them.
With that in mind, I don't think this really requires much prep work as a GM. The ball is very much in the players' court with this one; all you really need to do is try to spot any obvious flaws in whatever plan they go with as they come up with it, and make it fail if you don't think they've done a good enough job accounting for these flaws. If they're anything like most parties I've seen in such situations, they'll spend a whole session planning giving you plenty of time before the next session to think about how their plan will play out and where any weaknesses may lie.
