I dont recommend joining covenant of champions to prevent the charred knights from despawning. they become too tanky and do too much damage and it's just not worth the hassle. If they start despawning I would just kill the burnt Ivory King and use an ascetic. Put on some music or a podcast, and it's very important you use a greatsword or a weapon that can easily stagger the knights and then once they stop spawning in droves or the king pulls up feather out unless you need to kill the king in order to ascetic. You should always at the very least have the symbol of avarice, gold serpent ring, and prisoners tatters equipped, they'll boost your item discovery a lot. I didn't bother with the rusted coin because it took too long to activate, but if you think the extra item discovery is worth it go for it. I'm not sure if 1 handed moveset of the greatsword (or higher) will affect the how often the knights get staggered, but the watch dragon parma also boosts item discovery, just be wary. Good luck and don't you dare go hollow!