Two Wizard Challenge achievements deal with projectiles: Time Takes Its Toll (slow 20 with one Slow Time) and A Forceful Rebuke (reflect 8 with one Wave of Force).
It seems like getting both with the help of the same friendly demons would be pretty easy, but I can't find any that are willing to shoot me that much; gangs are either too small or fire too slowly. Where's a good place to get shot?