I was in a campaign some years back where the ultimate goal was to defeat a god. To "warm us up" the DM arranged partway through the campaign for us to get into a vendetta with a lesser demon, and it proved rather illuminating. When we went to fight it in its lair it creamed us because it had been preparing specifically to fight us, we barely teleported out with our lives. Then when we sat down together and started planning how to make another attempt, it teleported into our lair (a mansion owned by an NPC ally) and attacked us. We were like, "how dare it scry-and-fry us while we were preparing to scry-and-fry it!?"

Many high-level monsters have high intelligence scores, if the DM actually accounts for that then they can be almost arbitrarily powerful.

When we eventually went to fight the final boss god we came loaded for bear, we had something like 8 different supernatural armies on our side and we dropped an artifact superweapon on the god's domain as the opening salvo. It was still a mighty slog, though, and at that point in the campaign Balors were essentially considered just "footsoldiers" against our party.