Tsar Bomba
Tsar Bomba (also known as the floating bomb glitch) is a glitch that allows a bomb to be stalled in midair indefinitely during the Hariet fights in Bowser's Kingdom and Dark Side. The glitch is caused when Mario knocks both of Hariet's bombs back at her at around the same time; after the first bomb stuns her, the second will stall at the end of its trajectory and float indefinitely. The floating bomb can then later be used to stun Hariet or cancel her hovering attack even before she has released any bombs during that phase, which can speed up those cycles.
Execution
When entering the fight you want to roll and then roll cancel backwards. Then you want to throw cappy out and shake the controller to home in on the bomb to Mario's left, while shaking the controller Mario needs to be walking back so that way cappy doesn't go back to Mario and then just hit the other bomb. If done correctly the other bomb should be floating midair not moving at all and just spin there. After hitting Hariet in her hovering phase you can use the floating bomb to hit her by throwing cappy. When Harriet is hit jump and hit her in midair before she touches the ground. This speedrun trick saves roughly 2 seconds.