Easy drops R events.
Medium drops R/R+ event items rather commonly especially at the end.
Hard rarely drops URs but have not encountered UR events so far.
Edit - One R event found by ally... Holy Lantern.
Another guy on forum dropped a R+ (Upending Throw) in medium-12.
Edit2 - Just played Medium-12. SO. MANY. ARCHERS. Still no event card?! T.T