Nightmare

Nightmare is first encountered as the level boss of Mission 18.

Strategy

Vergil trivializes this with his destructive JJC and DG. This is the same with Dante on Cav.

This fight depends on how well you can hit Nightmare's head. If you fail to hit the weak point consistently then you may want to turn off DT at the third bar to min-max your Sword Formation DPS.
Shadow and Griffon will get into stalemate-state instantly thanks to DSD properties with Cav Air Combo B.
If Nightmare escapes into Phase 2 then you will lose 1m.
Vergil must activate Spiral Blades while approaching Nightmare because this will instantly cause Shadow & Griffon to stalemate instantly.
Similar to Mirage Goliath & Artemis, aim and release Spiral Blades at Nightmare's head for big damage.
JJC in SDT can hit all three familiars at once.
Sequential Judgement Cuts can save more time.

Dante New Game example

Dante New Game+ example

Vergil General example

It's good to check the Frequently Asked Questions (FAQ) if you have general questions regarding the bosses.