For all those wondering where is the instruction screen, it's accessible while you're in a stage, top right hand corner interrogation mark.
And to those complaining the sudden death challenges being too hard, here's what I do, just run/roll around in circle so all the mobs are chasing you then when they're quite aligned use the K or X attack then wait for it to reload and repeat, it takes a bit long but you evade all the range attacks and hit several mobs at the same time, buy damage rune to do it faster.
 "Tap Heroes"){kind=icon}
 "Clicker Heroes"){kind=icon}
 "Disaster Will Strike 3"){kind=icon}
 "Mighty Knight"){kind=icon}