Castlevania Advance Collection

Legion is an "Optional' boss, in that it and the soul it guards (Galamoth) are not needed to beat the game. However, you will need it for Aria's 100% Soul Collected achievement, and this boss is the reason said achievement is missable; if you do not kill all parts of it, you do not earn the soul and cannot get it at a later time.

IMPORTANT!

Save before the boss - Save room plus the Collection's save state feature. You MUST destroy the four 'shell' pieces protecting the core of Legion before beating the core. If you do not, you will not earn the Legion soul, and thus cannot get the All Souls Collected achievement. Use the save point to make sure you earn the soul.

Legion is accessible after you obtain the Skula soul, a reward found in the reward room after the Death boss fight in the highest part of the clock tower area. This soul lets you sink underwater, allowing you to explore more of the castle. Specifically, you need to get to the very bottom level of the map. If you've found the Arena area in the very bottom-left corner, Legion's entrance is a few rooms before it, going down and then to the right. When you see white doll/humanoids that exit from the wall, slowly walk, and can't be damaged nor hurt you, you are in the right area.

The fight is in two main parts:

1. Breaking apart the corpse shell protecting the core
2. Defeating the Core

Part 1 involves killing the 4 outer shell pieces. Each has 5,000 HP separately, as does the core. The general mode of the boss is to circle the boss room at a slow pace, attempting to damage you by touch. As it does, you need to attack the shell to destroy it. You can do so as it is moving horizontally, and then quickly move to the top/bottom as it moves along the left or right vertical portions of the room. As you attack, Legion corpses may fall and turn into an enemy. These take around 250 HP (estimate), move slowly, and only damage you if you touch them. They are low danger, but it is best to clear them out of the way before Legion gets close.

Part 2 is after all four Legion shell sections are gone, leaving the Legion core and its 3 snake attachments. This form can damage you either by the core itself touching you, or from the laser beam from the three snake heads. The snakes themselves will not harm you as they move, nor can you damage them directly. They will signal when they will fire, and it is a long beam in the direction it is facing. When the snakes slide back into the core, Legion will do a bounce attack, dashing at high speeds at an angle. Stand back to get a feel for the angle it bounces, then run/slide past it once you can. It will bounce for a distance, then it will resume roaming the stage with its laser snakes again.

Once you break all 4 pieces of the shell then destroy the core, you will earn the Legion soul. Continue to the right to also obtain the Galamoth soul. This will let you deal with the Chronomage rabbit that warps you out out of an area in the upper-left part of the castle. Chronomage also has a soul to collect, so do not forget that.