Once all of the tadpoles are dead, roll and/or jump attack towards Deku Toad until Link is near enough that the camera shifts and Deku Toad leaps to the ceiling. This is faster than waiting for Deku Toad to leap by itself.
Entering first person with C-Up once Deku Toad has fully reached the ceiling will cause it to fall sooner than it would otherwise. If doing this, exit first person immediately after the camera has fully shifted to Link's point of view and roll away to avoid being crushed.
When Deku Toad falls, it will always face the center of the room.
Having the early boss fights (EBF) flag active skips the cutscene of the door sliding shut that normally plays when Link enters the room from below, so the fight begins as soon as the player enters first person and looks up at the ceiling. This can be done from anywhere in the room, which speedruns can abuse by entering the room from the side door and starting the fight by looking upward without having gained access to the waterway below and blown up or clipped past the underwater rock. This saves 37 seconds in Any% and 7 seconds in All Dungeons.
It takes 10 hits from the Master Sword on Deku Toad's tongue to finish the fight. These can all be fit into one cycle, but the timing is very tight. A timer begins once the tongue is hit for the first time on a cycle, and to fit 10 hits in within this span, stand near the tip of the tongue and repeatedly do single or double slashes until the fight is over, starting each as soon as possible without initiating the third part of a combo or beyond.
It is possible to include the third part of a combo and still make the 1-cycle, and it is also possible to finish the 1-cycle successfully with a full combo, but these take extra time and are not recommended. Finishing a combo at any point will cause another cycle to begin immediately if the combo did not reach the 10 hit threshold. A running slash does not count as a full hit, so take care to stop moving sufficiently before beginning the slashes.
It is easier to 1-cycle Deku Toad with the help of a water bomb than with the Master Sword alone. A water bomb deals the damage of 5 Master Sword hits, so hit the tongue 5 times with the Master Sword and have the bomb explode shortly after. To time the explosion, pull out a bomb as Deku Toad falls down and hits the floor, and place the bomb such that the explosion will hit the tongue but Link's 5 slashes will not hit the bomb.
This can be done with a normal bomb or a water bomb. Immediately after the fight begins, pull out a bomb. As soon as possible, press c-up to begin entering first person, and before the camera fully pans in, press and hold L and tap the boomerang button to throw. If done correctly and quickly enough, the boomerang will target Deku Toad's tongue and pick up the bomb before it falls to the water, and Deku Toad will eat the bomb before its vulnerability period ends. See frame data.
This is the quickest method of killing Deku Toad, but it requires the Bow and bombs. Immediately after the fight begins, target and shoot a bomb arrow to knock Deku Toad down without spawning any tadpoles. With Master Sword, slash the tongue 5 times and shoot a bomb arrow to finish the fight. Targeting the tongue for this second bomb arrow shot is inconsistent because the bomb arrow will sometimes defuse in the standing water; manually aiming the bomb arrow at Deku Toad's lip behind the tongue is much more consistent.