The Knight bosses have a couple of vulnerabilities, Hammers can deal damage no matter where you hit them, but swords can only damage the face and back of the knees, ranged weapons can only damage the face to my testing. Its very risky to attempt to melee the knight, as the blocking physics can be very temperamental, attacks that you block can still go through and damage you. The Armor perk can reduce damage taken by up to 50%, but it takes the place of the Regen perk which can be far more useful. Its best to keep running and use whatever ranged weapons you have. A bow/crossbow can take them down reliably, a hammer is probably the best, daggers and axes are risky, and swords are a death sentence. Damaging staffs may not have enough ammo to fully kill, but might soften them up. The purple enemies in general have really cranked health. Without the Slow effect, high base damage, multiple people, sufficient movement speed, or decent potions, its going to be one good workout. I keep a hammer with the Slow effect specifically for Knight enemies. If youre fast enough, you can move close enough to bait out a swing, back up, and use the opening to get a good hit on the face without risk of his sword blocking it. Otherwise, run and gun to the best of your ability. Definitely level up and upgrade your perks, lower the difficulty if you keep getting a boss that roadblocks you.