Darknuts ? are these the armor guys or the plant ?
if they are the plant i cant see why you have problems with them xD
if they are the armor ones mmm one by one that was my strategy i use back jump when they use their jump attack and immediately use the normal jump attack (not the charged one) and back off and repeat the process
it takes time but is the best way in my opinion it`s a safer one and you take control of the battle
and when you destroy their armor i use then rolling jump (that attack that you get to the back of the enemy by rolling) to break their defense and attack like there is no tomorrow until they block you attacks immediately after that back off and repeat the process ;D