How difficult is Whimsyshire?

Solution 1:

I have only experienced Whimsyshire on Normal difficulty, but the monsters there were on par with what you face in Act IV. I brought a level 24 friend along and he was getting stomped pretty hard (but not oneshotted). I, as a level 32ish wizard at the time, found the creatures appropriately challenging to my level.

I would assume that in higher difficulties, Whimsyshire is similarly scaled to be roughly equivalent to Act IV of that particular difficulty.

Solution 2:

If you can do the difficulty higher on Act I with minimal problems, I'd say you are ready for that difficulty of Whimsyshire.

So if you can take Nightmare Act I, you are ready for normal Whimsyshire.

Note that with a group of 4, this can change drastically. Soloing will not be a problem at this point however.

Solution 3:

Also note that there are no ranged monsters in Whimseyshire, aside from possibly the Mortar mod on elites. Depending on your character, this might make it significantly easier than similarly-leveled areas.