If you mean what difficulty are runs done, than any difficulty works really. Generally we run easy though.

if you mean how hard is the run, then the answer is that it isn't difficult to understand the concept, there aren't any crazy glitches or tricks that need to be preformed. The difficulty lies more in RNG and reflexes in combat. You can play very safe and easy with plenty of precautions, or very risky and have more chances of losing a run.

Hope that helps!