Walk forward once the fight starts and throw the boomerang right before you jump off then pull your sword out in midair. Do one roll out of the fall slightly right then instantly jump attack Dangoro, he will then curl up and you can sidehop to equip the Iron boots on faster and throw him in the lava.
Below is a video of fighting Dangoro with roll stabs for all the hits. The first hit is slower than mentioned above but the second and third has audio ques to listen for.
For the second hit wait until the first beat stop playing.
For the third hit wait until just after the second beat starts playing.
If successful, Link will grab the ledge after throwing Dangoro, and Dangoro will bounce in the lava enough to complete two cycles instead of just one. If Dangoro is grabbed too early, then this will act as one normal cycle; if Dangoro is grabbed too late, then Link will fall and grab the ledge without throwing Dangoro into the lava.
While he bounces after the second cycle hits start, hitting him in midair with a spin attack will interrupt him for a quick third hit.
Original video [Wii]: here
The same effect is achievable with Ball & Chain. Jump attack from the spawn position, then throw the Ball & Chain immediately to make Dangoro start rolling. Put on the Iron Boots and roll straight down once. Tap up to make Link face Dangoro, and time an A press to grab and throw him.
With the Ordon Sword equipped, three jump attacks don't reach Dangoro, but it is still possible to throw Dangoro from the correct position to get the instant second hit. Below is an example with a Boomerang stun on the first hit, then a sidehop and backflip into position to be knocked backwards by Dangoro's rolling attack.
After receiving the Instant Second Hit, Dangoro bounces on the lava for long enough that there is time to attack him before he lands again, forcing him to land and immediately begin his roll charge attack. This saves an additional 3 seconds over allowing him to land regularly, but he doesn't always bounce close enough to Link for this to be possible.
For categories that have shield bash during this fight, it is faster to get Dangoro to expose his belly with a shield bash than by waiting.