Personally, I prefer instead of the B-team to have a single stronger character. Basically the character of "he is so much stronger than us and he still could not take the BOSS".
It doesn't really take away from the parties heroic nature since due to the nature of DnD, the party is almost always more effective than a single stronger character.
It lets me have a really strong Deus machina in case the party is losing very badly.
Also, I just prefer the party not be the typical heroes where all the world is rooting for them to defeat the demon lord, or something dumb like that. Rather I would have the real hero fail and the party then have to take up the mantle.