Lavos opens with a physical-based AoE (“Destruction Rains from the Heavens!”), then casts Obstacle/Chaotic Zone (the effect of which is randomized every seed, but is consistent with what the TerraMutant and Mega Mutant cast), and then a non-elemental AoE (Energy Release). After this the cycle repeats.

It's highly recommended to protect against Obstacle with the appropriate gear, especially if you don't know what random status effect is coming. This is a relatively simple fight, but a single Obstacle has the potential to stop the party in its tracks.

Inner Lavos begins the fight with “Laser Beams: Doors of Doom Open” for moderate magical damage to all party members. Quickly after, the arms will follow up with a single-target physical attack each (these are nicknamed “boomerangs”). The combination of these attacks will down the targeted character in all but a few exceptional circumstances. It's highly recommended to heal the entire party right away unless you are able to defeat the arms with your first actions.

Regardless, dispatch the arms as quickly as possible, ideally at the same time. They can inflict Stop, and if one dies before the other, the remaining one will cast Protective Seal, which negates positive status effects and status protection (even from equipped items). They can also heal the body, but as it begins the fight with its defenses maxed out, you won't have dealt much damage for them to heal. If the arms remain alive for too long, they will continue to throw boomerangs and wake the main body, bringing more damage upon the party.

Once the arms are down, Inner Lavos will begin its battle script. Wait for it to cast Obstacle, as it will not drop its defenses to normal until then. After Obstacle, it will cast Shadow Slay (which inflicts Poison), then Flame Battle (a single-target Fire attack). The next attack is Shadow Doom Blaze which inflicts high magical damage. Ideally you should defeat it before it uses this attack.

After Shadow Doom Blaze, Lavos will use Evil Emanation to increase its attack power and repeat this cycle.

The Left Bit heals the Center Bit, and will gradually strip out the status immunities of the party. The Center Bit is the main attacker, and can deal very heavy damage. When it time warps, it will change its attack pattern, and can start a chain. This chain is broken after killing the Center Bit.

If the Center Bit goes down first, the Left Bit will start countering. Killing either the Center or the Left Bit will lower the Right Bit's defense. The Right Bit revives the others to full health, but its defense will remain lowered for several turns. If Lavos Core starts casting Invading Light, defeating either the Left or Center Bit will cancel the attack.