From what I've read "cruel and unusual punishment" is less of an AND statement about punishments being cruel and non-standard and more of a double descriptor of the cruelty with the unusual portion more focused on a sort of terrible inventiveness and also, IMO to stand as a sort of fallback for questionable punishments, if the punishment is innovative it would need to be proven to not be cruel before it's application - and the US has (IMO) failed to adhere to this standard in the past with some methods of administering the death penalty - specifically the electric chair.

I personally don't like death penalties (due to their misuse as a political tool and the general high error rate of judgement) but I believe proper lethal injection fits perfectly within the bounds of not being cruel nor unusual.

So, if someone said that lethal injection was more humane if administered in direct sunlight while the injectee was hanging upside down due to blood movement some-such... assuming they were telling the truth about the efficacy then it'd likely pass the 8th amendment.