This is how you unlock Bandit's new special skill, desperado, by completing the Bandit B&E challenge in risk of rain 2.
First, the same as the other alt skill unlock guides, use artifact of command and stick to drizzle difficulty. It is way faster and easier this way. Also, optionally, you can get artifact of glass.
Second, stack glass, ap rounds, and other damage boosting items that ONLY impact damage and aren't on hit. Apparently runald's and Kjaro's can mess up the achievement and cause you to not unlock Desperado.
Third, use lights out to last hit Mithrix. It really is that simple.
I think desperado is pretty cool (even though at first I thought it was bad). Let me know what you think and I hope this video helps!