In phase 2 of this fight, you normally have to wait for Ook to bring bomblings to use on Diababa. But, instead, you can just juggle bomblings from phase 1 to avoid having to wait. Doing this requires good positioning and fast targeting. This trick saves over 30 seconds compared to doing the fight normally.
Note: When a bombling is inside the boomerang, its timer resets. This is how we can keep bomblings into phase 2.
Kill the Diababa head on the left normally, then run up to the edge of the water in the middle of the room while waiting for it to die.
Boomerang the two left-most bomblings (your position here matters so the bomblings will be in front of you at the start of phase 2).
While waiting for the bomblings to come back to you, cycle through targets until you target the bombling to the right. One of the bomblings should end up in your hands, and the other will be at your feet. Pull out the boomerang (hold the button) and keep targeting the bombling on the right. This should drop the bombling in your hands. Then walk a little to the right, target the Diababa head on the right, and let go of the boomerang.
During the cutscene, hold target and down. As the cutscene ends, tap the boomerang button to instantly throw the boomerang at Diababa. This will send both of the bomblings from phase 1 towards its face. If this doesn't work, your positioning was wrong in phase 1.
While the boomerang is going towards Diababa's head with the bomblings, roll back to the right side of the fourth plank from Diababa. Then target Diababa again and wait for the bomblings to come back to you.
One bombling will end up in your hands, and the other will be at your feet. Throw the one in your hands towards Diababa, then backflip.
Target the bombling nearest to you and then target Diababa's head (you should be able to automatically target its head without having to aim, but you'll need to aim at the bombling on the ground). The bombling you targeted will hit Diababa in the face, and the one closer to Diababa will get caught up in the bottom of the boomerang. If the second bombling blows up or doesn't get caught in the boomerang, your positioning was off.
C-up and look over to the right, then throw the boomerang once the bombling comes back to you. Then pull out your sword, walk a bit towards Diababa, target its eyeball, and jumpspin.
C-up and look up at where Diababa's head will be after he finishes flailing around. Once the other bombling makes it back to you, wait about a second and then throw the boomerang. This should cause the remaining bombling to hit Diababa's face.
Back up to the left side of the third plank from Diababa while targeting its eyeball (you will lose the target at some point, but that's fine, just keep holding target). Right when the eyeball is about to hit the ground, do a jumpspin to finish Diababa off.
If you're doing the fight normally, Ook takes a long time to come out with the second bombling during phase 2 because Diababa falling down delays Ook's timer a lot. But we can save a decent chunk of time over the casual method by cooking the first bombling a bit before knocking Diababa down the first time.
Quickly target all three Diababa heads, then throw the boomerang (you can just mash the target button with the boomerang out). Ook will not come out until both of the side Diababa heads have attacked you, and this causes them to attack faster.
Boomerang Ook's first bombling, and wait in the back-middle of the room for it to come back to you.
Throw the bombling forward, wait for a few seconds while targeting the bomb and Diababa's main head, then throw the boomerang. If you waited long enough, Ook should come out with a second bombling.
Jumpspin Diababa's eye, then target Ook's bomb and Diababa's main head, and throw the boomerang again. If you timed everything well, the boomerang should pretty much go straight towards Diababa's head, picking up Ook's second bombling along the way.
Back up to the left side of the third plank from Diababa while targeting its eyeball (you will lose the target at some point, but that's fine, just keep holding target). Right when the eyeball is about to hit the ground, do a jumpspin to finish Diababa off.