"... YOU WILL NOT SLAY ME!"
Amaranth summons knights to fight for him and just generally be obstacles in your movement around the field of battle. He also teleports occasionally off screen, reappearing with a charge attack already charged up and rushing at the player with his sword. Amaranth's sword is his primary means of attack, and he uses a couple different techniques to attack. If the player approaches Amaranth too closely, he will attack with a simple, stationary sword
He also has two variations on his rushing charge: he either charges up for a long dash across the field, or performs a chain of three very short-range rushes- he can change direction between each portion of his triple rush.
Amaranth's knights are pushovers, largely because he can hurt them himself, and break their shields, with his attacks. His attacks have a fairly large range for a melee sword, so he can hit multiple knights in one attack- they only take a couple hits from him to go down. There is an endless supply of knights, but they are a fairly minor issue overall. When Amaranth teleports, the only thing you can really do is run laps around the arena until he shows up, and then run directly away from him. Amaranth's proximity swipe has a fairly slow reaction time, so getting a melee attack or two or three in is not impossible if you rush at him between his special attacks.
When he begins one of his two charge attacks, the best action to take is to simply flee as far away as possible. If he is in a corner of the screen, go to the opposite corner, because while his charge attacks cover quite a distance overall, they aren't all-reaching. Finally, it is really recommended that you use ranged attacks or abilities if possible to keep distance from his greatsword for as long as possible. If you're incapable of doing ranged attacks, careful positioning and strategic attack timing can work.