For some reason it is possible to reset the timer by only doing the first 3 hits of a combo attack. If you do all 4th the damage threshold will start and if you do 2 then the timer doesn't reset. So the idea is to chain 3x combos with little gaps in between them to allow the timer to reset.
To kill him off quickly you should jump attack him once you knock him down as his timer is long and allows for the extra damage, after that chain 3x combos and finishing the 6th combo set to kill him. The Jump attack is kinda risky timing-wise though so you might want to skip it and just keep slashing.
Death Sword has two conditions which cause him to fly away after being knocked to the ground and only one of them has to be true for him to fly away.
The first is a timer that starts counting down once you hit him for the first time, however there is a longer timer if you don't hit him at all but that's unimportant.
The second condition is a damage threshold, when you deal 2 critical moves such as a jump-attack, spin-attack or the 3rd and 4th hit of a combo.
The One cycle abuses the fact that the timer doesn't run down as long as you are slashing you sword, regardless of weather you are hitting him or not.
It is possible with a precise wolf superjump to reach the ledge above the gate in the spinner room. This makes it possible to skip the second half of the Death Sword fight. Skipping Death Sword saves around 18 seconds, but missing the superjump loses about 25 seconds.
After using the clawshot on Death Sword, roll into the corner and wait for it to land near you (hope it doesn't get stuck). Roll down once, press B to slash, then transform immediately. Hold B to start charging and walk down to the position seen in the video. Depending on position, there is between 1-5 frames to release the charge.
!!!This trick is very precise. Attempt at your own risk.!!!