Bandit Keith lv40 is almost impossible beat and so freaking annoying. The only "legendary duelist" i have to fight at lv20 to not throw my freaking phone through the window
Bait him to use at least 2 of his 7 completed on the same monster, then use monster removals like Tribute to the Trash, Mirror Wall if he hits you etc, simple. Just watch out for his Sergeant Electro shenanigans.