Two Boosters + Five Streak (odds of losing it)
Normal = Basically none
Hard = Basically none (unless incorrectly classified)
Super Hard = Small chance to lose streak (unless boosters break some mechanic at the start of the level)
Nightmarishly Hard = 30-40% chance to lose streak
Legendary = 20% chance to lose streak
Double Boosters (to pass a level without it being lucky)
Normal - Not required
Hard - 50% of the time
Super Hard - 90% of the time
Nightmarishly Hard - Only a few mechanics levels can be passed
Legendary - 100% of the time