Various enemy visual tweaks

Most late-game enemies have been removed from regular arenas and are instead accessible in the endless mode.

Greatly reduced the number of crystals needed to unlock the boss fight

I fixed a bug where current mythic text would appear on the reward screen even if there was no current mythic.

Amulet chests now drop from regular enemies instead of OSIRIS

OSIRIS now shoots his bullets from above down toward the player. These bullets attempt to predict movement.

Adjusted enemy health scaling to 0.625 to 1.625 over difficulty levels.

(Game was reworked around 1-minute matches, but that's been scrapped)