Attempting random encounters in DLC 3 (and the game in general) is a mistake. If you really want to kill them,
mana drain the knight
Yo, dude, you know your stuff eh?
Edit : for the DLC 3 strats, back then i just used
Miranda's Axe (certain hit),
Mary's Wand (certain hit + hit all), or Morningstar (hit all) for main weapon, while using Church Rifle for additional damage, with additional action in a turn (White Rabbit Ring, Black Rabbit Ring, and/or
Mabel's underwear, similar rings' effect will stacked), better yet, get
Lindamea's ring too (for status immunities) later, it's still useful even in
endgame, and
Sho's covenant for percentage damage reduction.