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. The strategy is slightly different depending on weather you have the ball & chain or not.
For 100% and any other situation where you don't have the Ball & Chain 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
For Other categories with the Ball & Chain, once you knock him down you should swing the Ball & Chain to hit him 3 times again to deal more damage while the timer is high then chain 3x combos and finishing the 4th combo set will kill him
The following "setup" by SVA is the fastest possible way to kill Death Sword. The timings on this are pretty strict.
After attacking Death Sword as a wolf and causing him to materialize, hit him with the boomerang to aggravate him. Now pull out the Ball & Chain and keep it spinning above your head while Death Sword moves around the room. When he gets hit, move a bit closer to him so the Ball & Chain will hit him. Hit him with it 5 times and then use the boomerang to aggravate him again. Repeat the process once more to finish him.
With really precise movement and good RNG it is possible to skip the entire Death Sword fight by using a wolf Super Jump.
The setup is frame perfect though and therefore not recommended for RTA. The timesave is about 15 seconds.
Here is a decent setup made by Samura1man. 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
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.