So the players are exploring a wizard tower and I've got all of it filled out except the final encounter,  which is a riddle spitting monster of some sort but im not sure what. 

It's a party of 7 players at 2nd level.   They have healing but may not have time to rest before reaching this final encounter.  I was hoping for a minute demon of some sort with some interesting but not necessary deadly abilities. 

Any ideas?

A spectator can always make for a good riddle-giving guardian.

If you're looking for something a bit more fiendish, a green hag in league with the wizard could create some riddling mischief, while also not being too deadly. 

Woah, spectator looks awesome!  Thanks for the suggestion!

The spectator is a great one and I didn't even think of it. I must train harder.

An answer I did come up with is to use around 4 (2 each) of a mix of specters and shadows to weaken the party before the final showdown.

You could use a gibbering mouther and adapt its characteristics to suit your story. And if you need a little more kick to it, you could have a rug on the floor animate at the right time and turn into a rug of smothering

Another fun option is to skip the monster and go for a Magic Mouth and trap (that triggers if they don't answer the riddle correctly).