Walkthrough
Leviathan's Stomach
After Dante gets swallowed by Leviathan he drops into the demon's cavernous stomach. This map is a lot more open and non-linear than most maps; to proceed, you must release the five tusks that seal the door by the wrecked bus on the ground floor by striking several bulbous switches throughout the stomach. Be careful though; stepping in the pools of yellow stomach acid will damage you.
The first is right on the ledge right next to where you land. Hit it, then continue along the ledge; the next one is ahead beside the ship. You can wall-jump/Air Hike onto the edge of the prow of the wrecked ship ahead to find yourself a hidden Red Orb cache, then walk onto its deck to find a Blue Orb Fragment. Jump down to the ground floor; you can see a door behind the breakable hull of the ship that leads to the Secret Mission, "Devil's Teetor-totter". You need to ride the elevator to the top while fending off DT'ed Hell Prides so that the elevator won't overweight and fall down. To complete this secret mission easier, wait until you have Beowulf and its Volcano attack or Kalina Ann. Use Beowulf's FULLY charged Volcano to take out all the Hell Prides easier. After finishing the secret mission, double back toward the wrecked bus to find the third one, then jump over the bus and head into the hole on the right. The fourth is ahead in the first cavern behind the acid waterfall. Hit it, then jump up through the next hole, past the next cavern and across the stone bridge. A Red Orb crystal is to the left, and the last switch to the right. Hit it and
Two craters on the moon form "8", depicting mission's number.
jump down the ledge to land on the bus beside the door that is now unlocked, leading to Leviathan's Intestines.
Leviathan's Intenstines
There are puddles of acid here too. Hug the right wall to avoid most of it, although there are two parts you must jump over to avoid stepping in it. Near the end, you'll find a Divinity Statue partially inside the right wall.
Leviathan's Heartcore
You'll arrive at Leviathan's Heartcore, but you can't do anything here yet, so continue to the next Intestines map.
Leviathan's Intestines
A Gigapede will appear behind you after you take a few steps. Killing it is optional but will earn you 1,000 Red Orbs; if you choose not to, you can just run down the map where you eventually reach a section that the Gigapede cannot pursue you into. Head into the next segment of Intestine to encounter a new type of demon, the Hell Envy. While they behave similarly to Hell Prides, they differ in that they typically can't be knocked down even by end-combo hits (you can only make them stagger), so keep an eye out when dealing with groups of these. You can ignore these if you'd like, though, and head through the map to Leviathan's Retina.
Leviathan's Retina
Why an eye would be connected to an intestinal tract is known only to Capcom. Regardless, a group of Envies spawn in this room too, but the key item in the center of the map, the Ignis Fatuus, will absorb all orbs that they drop. That's fine, though; after it absorbs enough, it becomes exposed enough for you to pick it up. Taking it will unseal the way to the next section of Intestines. However, taking it will also disable all lighting inside Leviathan.
Leviathan's Intestines
There is no Gigapede in this map, but there are two side branches. The first contains a Green Orb and the second has a Cerberus Combat Adjudicator that requires an SS rank. The Intestines map after this one DOES have a Gigapede, and leads back to Leviathan's Stomach.
Leviathan's Stomach
Envies appear here too, but are again optional. Head back into the door that you unsealed when you first started the mission.
Leviathan's Intestines
The lights here are out too, making it difficult to see where you have to jump over the acid puddles. You can fire Ebony & Ivory to briefly illuminate the area. Once you've reached the Divinity Statue, prepare to fight Leviathan's Heart.
Leviathan's Heartcore
Examine the glowing green "pedestal" along the wall to place the Ignus Fatuus and dissolve the membrane protecting Leviathan's heart.
Boss: Leviathan's Heart
First playthrough
This fight consists of three boss parts: The right heart, the central heart, and the left heart. The central heart is the target that you must destroy, but it is covered by a shield most of the time. To make it expose itself, you must deplete the HP of one of the other hearts.
The catch is that by attacking one heart, you leave the other one alone. The right heart will drain your DT gauge if not attacked for a few moments, signified by white lights being absorbed from Dante; the left heart will absorb all Orbs that are dropped. Hell Envies also constantly appear on the map, making attacking the hearts more difficult.
The first time you destroy a side heart, the central heart will simply expose itself for a few moments. All subsequent times, it will counterattack first (which does expose it), telegraphed by a red or bluish glow around the heart depending on which heart was left alone before exposing itself again. If it has absorbed enough DT, it will release purplish spores that float slowly toward you; if it absorbed Red Orbs, it will sweep a red laser beam from side to side (two beams when it has sustained enough damage). Both attacks are capable of killing any Envies that are caught in them. Attacking the right heart is recommended, since keeping your DT increases your damage output, and causes the main heart to counter with lasers (which are both easier to avoid and highly effective at killing off the Envies).
- For those who use Swordmaster: Bring Agni & Rudra (the hearts are weak to fire attacks) and select Swordmaster. Jump up beside the right heart and begin using aerial Swordmaster combos to stay airborne (e.g. first two hits of Sky Dance alternated with Aerial Rave). This minimizes your time on the ground and thus the risks of getting mobbed by the Envies. When the heart dies, the main heart will counter with the lasers - you can jump over them or hop up beside the main heart and use the aerial combo method to deal extra damage. When the heart closes again, the right heart will have regenerated; repeat this strategy until the central heart dies.
- For those who use Gunslinger: It is advised to bring Artemis and get to at least Level 2, while a recommended secondary would be E&I. While keeping Style in check, using Sphere into an Acid Rain is capable of eating away at large chunks of life to the hearts. It is advised to keep away the Envies before starting up.
Second playthough
Combining Nevan's aerial Swordmaster move with Agni & Rudra's essentially eliminates having to worry about Envies at all, since Air Slash and Sky Dance combined altitude loss is not enough to drop you completely to the ground before the right heart dies. Air Slash also does appreciably more damage.
Alternatively, you can use Tempest/Ultimate Tempest on the side hearts (and the central one during its laser attack if you're feeling daring and can mash buttons quickly enough) to quickly take them out.
If you find yourself in a mob of Envies, Kalina Ann's blast radius is large enough to hit all of the ones close to you, allowing you to jump away before they can attack again.
Defeating Leviathan's central heart will automatically complete the mission.