You can also have to opposite problem I had a professor giving a test so easy that the worse grade was 17 out of 20. I would prefer an almost impossible test and then be graded against my classmates, you just have to provide some rules against gaming (minimum requirements for example).
I had one professor who would pick the best score for each short answer and then then grade everyone else based off of that score. Of course she also had a rule that if the best wasn't good enough she could grade against her own standard. It wasn't probably the most scientific way of grading but it was interesting. She also pretended to be entirely blind to who took each test, so everyone got a random number ahead of time and then you went to talk to her about the test and told her your number, and she'd give you that test. You could then look at everyone's test to compare them. During this conversation she would refuse to admit that it was your test and just pretended like it was something you were interested in looking at.