You can’t twin spells against the same target. Quickened spell is more likely. However imo “going nova” against a single boss is part of the fun. You’re right that having multiple opponents is the best way to go. But 5e has good mechanics available to make them feel like multiple enemies.

Big ol’ sack of HP so they don’t drop round one

Legendary resistances to protect you from save or suck spells

Great mobility to protect them from getting flanked

Legendary actions to mimic multiple opponents

And finally some sort of AOE to break the party’s formation and break the casters concentration.

Batta-bing-batta-boom. You’ve got yourself a good ol boss fight.