Heart of Leviathan

Heart of Leviathan is encountered as the level boss of Mission 8. The primary goal for this fight is to kill the main Heart before or during cross laser pattern.

This boss in Mission 18 is optional.
Right Lung takes 20% more Fire Damage
Left Lung takes 20% more Ice Damage

Strategy

Vergil trivializes this with his Yam Aerial Rave + PS. Dante must have full Rage-meter RG before entering the fight. Although, there are many dynamic situations that can occur due to Envies movement.

Only Dante can kill the Heart before cross laser pattern using using Aerial Cross JC's.

Although, it is not always guaranteed because sometimes one of the envies can be too passive on approaching the player.

This case is usually possible when attempting BanHeart or sudden double damage surge from RG + DTE.

Stinger to the Right Lung then Shotgun cancel. Vergil will have to do Trick up first before starting to Yam + PS because this allows him to land multiple Aerial Raves at once.
^{Old Heart} SC until Envy is nearby Flux Release. Keep SC'ing till Laser comes out. Aerial Cross into Air Release .
^BanHeart DTE to damage the heart and kill one envy. un'DT then SC till one of the Envies are close enough. Gold Release then start charging for DTE while SC'ing. If you are confident about the kill before laser pattern then release at 4th Stock otherwise save up for the kill during laser pattern.
^Vergil Keep the Lock-on at the heart and spam PS Aerial Rave. The overall damage depends on PS hitting the boss instead of envies but at least 2 envies needs to die to trigger the laser pattern quickly.

It's possible to dodge the laser while maintaining the DPS with delayed Aerial Rave and well timed jump.

This is an example of Flux Release granting more damage than it usually does.

It's important to remember that ^BanHeart release varies on DT gain so, sometimes it may not grant enough symbols back to charge for DTE kill. Therefore, Dante will have to opt for ^{Old Heart} ending.

MUST DIE

Same approach but, it's advised to avoid Envies as they cannot be knocked down. Vergil can completely avoid them with Trick up while attacking either of the Lungs. Dante can attempt to JC with A&R although, it's risky.
It's also recommended to attack both lungs or DTE on the right one to prevent it from stealing DT.
Guard and release the lasers. Vergil can low profile this with well timed Judgement Cuts.