Well, the easiest thing to do would be to just stab someone with a really good icicle, hehe....
No evidence, no prints, and if not found real soon, would put off the time of death about two hours...
Of course any real assassin wirth his salt knows that the best assassinations are ones that look like suicides or accidents.
With a gun, of course, you're usually opting for suicide...so there's another option with ice...
First, you capture the victim and drug them up with some fairly untracable muscle relaxers (they do exist) or sedatives...(if need be, you can even
match some trace elements to food served as a "last meal"

). Then, you have a carved pedestal made of dry ice (to hold the gun at a believable
angle), as well as a piece of dry ice between the hammer of a revolver... You can go somewhere, have an alibi, and then let the dry ice melt (leaves
no water), gun goes off, kills victim in chair (after forcing victim to previously fire gun of course, after the relaxants, and in another
location...this will give the proper powder burns, etc. and fingerprints), and rest of ice melts into vapor... Looks like the victim shot
himself...

Especially well if a note is added, but that could be tricky....