Multivariable calculus: hard problems with solutions

Solution 1:

Usually Schaum's outlines are a good source for lots of problems with solutions, in this case

  • Elliott Mendelson: Schaum's 3,000 Solved Problems in Calculus

and

  • Robert Wrede and Murray Spiegel: Schaum's Outline of Advanced Calculus

come to mind. I don't know if that's hard enough, however :-)