The Deep Lurker appears before the party as they explore Mysterious Unison and is holding one of Claves's Soul Shards. It is a palette swap of the Root Lurker fought in the depths of the Mandolin Church Catacombs.
The normal physical attacks are a pop hit and slamming attacks on a target and those nearby at melee range. Deep Lurker has a roster of four powerful Special Attacks. Its most dangerous move is Voyage to Pain, in which it charges through targets in a line. It will often follow this up with a normal physical attack or another Special on a target from behind. Fatal Bite is three bites on a single target with a chance of knockdown. Peek-a-Boo is a popping attack on a single target with a chance of Passive status. Finally, Nasty Roar is an echoing roar that hits all targets close by.
Deep Lurker's HP and general statistics are on par with the bosses of the Double Reed Tower of Sand and its attack pattern is similar to that of the Root Lurker from the Mandolin Church Grotto, so if the player remembers that boss's attack patterns, then this boss really isn't all too challenging. The only potential difficulty is that this is the first boss that must be battled at Party Level 6, and this can be slightly problematic if the player isn't yet entirely used to the Moving Commands mechanic. This can be alleviated by spending some time battling the bosses in the dungeon prior to this battle, which generally provide good EXP.
Build up a set of 32 Echoes before the battle and bring in a Very Odd Chocolate and Sock Incense item. Fill the rest of the item set with Angel Trumpets and Goddess Bouquets, and maybe a couple of Clover items if only using one weak healer. The Deep Lurker will probably get the first turn unless the party is very overleveled, but after that turn, put the party in position to attack and once everyone is ready, use a Special Attack and follow it up with five Harmony Chains. Some of its attacks can even be Counterattacked fairly easily if it doesn't inflict Passive, so it might be worth wearing equipment that protects against this status.
Viola can be useful as usual by using Hawk Eye to mark the boss for damage and can also easily target this boss with her normal arrow attacks. If Serenade is being used, she can quickly deal some massive damage to this boss with her powerful Special Attacks fully strengthened by Echoes. Otherwise, strong close-combat attackers like Allegretto and Jazz are useful for this boss.
In Encore Mode, no other special strategies are required, but levels of at least 50-55 are recommended, though a skilled player may be able to defeat it at lower levels. It may also be a good idea to have two healers in the party to relieve the pressure of having to rely on items to heal.
If Claves is sent into this battle at the level she was at when she rejoined the party, assuming she wasn't used at all during Chapter 3, she will gain a dozen or more levels if playing the Xbox 360 version, or around 9 levels if playing the PlayStation 3 version. If choosing to level Claves and she is not already at least 35 or so, do not keep her in the main fray, but instead have her retreat to a light area on the edges and only use Unicorn Horn. Equip her with HP and defense increasing accessories prior to the battle.
- In the PlayStation 3 version of Eternal Sonata, this boss's HP was reduced by around 45000 points and the EXP granted for defeating it was reduced by more than half.