While I certainly would avoid the ham-fisted hobbling of older editions (usually by denying or twisting the useage of spells), sometimes using the environment to put a monkeywrench into the straightforward approach makes things interesting. I think for a lot of these high level encounters that the environment is going to have to be an enemy/factor as much as the physical foes to challenge the PCs. I'm not just talking about putting the fight into a field of burning lava that deals damage but that hinders/limit movement or actions, changes the effectiveness of certain attacks or abilities or mucks with PCs ability to choose their targets.
In short, make the combat into a puzzle so novaing & focus fire can't be the go-to tactic.
While I don't play much high level, I much prefer using numbers than attempting to "top out" and overwhelm the PCs through superior power directed into a single (or dual) opponent. Outnumbering the party goes a long ways towards splitting firepower and resources. I also think it makes an interesting encounter if the PCs have to make (hard) choices on where to direct their firepower and who to take out first and gives the Leader/Boss/Subboss some ablative armor to let them do their thing before being taken out.
Doesn't even have to be that; Demons & Devils that aren't on their home plane return and reform on their home plane if defeated, and they can hold quite the grudge. If we were to use the scenario I suggested earlier, it could be the Pit Fiend and Balor are watching from a Yugoloth's "gaming" abode in Gehenna (neutral ground), for example.