How to beat Phalanx
Phalanx's spongy body is protected by blobs wielding shields and spears, so you'll need to remove them to make the boss vulnerable to your attacks. These blobs will separate from the main body throughout the fight, so try to take on them one by one and you will slowly but surely create an opening for yourself. Their shields mitigate frontal damage, so try hitting them from behind. The same goes for Phalanx itself. Try to hit the spongy core when it's exposed, hitting the shields won't do much damage.
Phalanx is also weak at shooting, so throw firebombs at him if you have any left. There are firebombs scattered throughout the level leading up to this boss fight and there isn't much reason to use them so far, so you probably have a few in your inventory that you could use. . Firebombs deal splash damage, so try to hit as many drops as you can with a single throw. Aim for Phalanx itself and you'll be sure to burn a few attached drops. If you have access to magic, you can cast Soul Arrow to speed up this fight, as the shields of blobs are not resistant to magic damage.
Once you have removed enough drops, the soft core of Phalanx will be exposed. You don't have to kill all of the blobs, but you will have to defeat enough to create an opening so that you can reach the spongy interiors of Phalanx with your sword. Once exposed, Phalanx takes quite a bit of damage and should descend quite easily. Beware of spots that might surprise you. If they cluster on top of you, you may not be able to get to safety.
After defeating Phalanx, you will receive 1270 souls and a main demon soul. You will also win the "Phalanx Trophy" trophy. An Archstone will appear in the now safe boss arena, and you can use it to return to the Nexus and meet the Maiden in Black.
Souls of the demon is now available exclusively on PlayStation 5.
