Nidhogg is one of the only boss that encountered once as the level boss of Mission 4.
No Damaging this fight is optional but very useful in Devil Hunter where as it's one of the biggest requirement for V to get Guillotine His sequence in NG has been tested throughly by GarudaKK
It's possible to delay killing the heads in order to get more damage and 1 cycle him in Human
The idea is the same but this time V has more in control with his familiars :
New Game+ cycle
This whole fight depends on your execution of JJC.
The DG which Vergil should be using is the Haste mode this allows him to instantly kill all of the Nidhogg tentcales while performing JJC. Some notes:
Cycle Example