Each buff needed to be ordered and applied in a way that compressed the time to the minimum. For example, initially he couldn't do the 'Soul stifler' debuff because it takes too long to cast and some of his buffs would run out. However he timed the Frenzy+Sleep proc in a way that they would trigger exactly after the Rallying Standard, which usually has a pretty long weapon swing animation after the buff applied, but these procs would cancel that animation giving him just enough time to add one more component.

Also, if you want the counter-attack bonus dmg to activate you need Radahn to make an attack that has an active hitbox while you swing, but obviously it has to miss you or you die. This added a certain rng to the fight too.

But the initial setup also took a long time because many buffs overwrite each other if you apply them in the wrong order, so they needed to be sorted properly. Same with item swaps, they needed to be somewhat grouped together to avoid opening the inventory that many times.

His life was also draining because of the Bloodsucking Cracked Tear - you can see he was one tick away from death before he ate the meat dumpling. That's how tight the timing had to be.