There is RNG involved in phase one and takes getting used to. The first hit is free, the 2nd and 3rd hit however can be harder. A tip is to mash A to jump off the rail immediately and hold right once you do, this helps because after hitting Stallord you always spawn to the right of him, heading towards the way he is facing. By doing this you can avoid his hands and fire and don't have to go all the way around him before trying to get at his spine
During Stallord's 2nd phase it is possible to time getting onto the rail as he shots a fireball at you, done with the right timing this allows you to catch up and knock him down without going all the way to the top.
To do this roll quickly 5 times staying close to the wall, adjust your camera to face the pillar after the 3rd, 4th and 5th roll and then get on the spinner after a bit of walking (listen to music - use spinner on the third note of the upwards movement of the tune).
If done right you will be side by side with Stallord and can just jump into him. If he shots the fireball but you didn't catch up to him, it means you got on the spinner, or did your last roll too late.
It is possible to 1 Cycle Stallord during his second phase by chaining spin-attacks together. You can slash him once or twice by mistake and still get the one cycle.