2 important questions.

[KH2destiny]

1.) Is the knight using elemental damge or just phsyical attacks
2.) If you were attacked by a lightning or fire heartless and it gave you an elemental attack of 20 pts from your hp. If you used defense boost would the attack be less effectice or do you just need elemental defense (ribbons and chains).
I'm at level 72 with 56 strength and 85 defense. I've been collecting alot of defense boost to up my defense. But no matter how many times I've upgraded the knight seems to do the same damage to me. Also I'm in critical mode just to let you know. I die after four or three hits from him and 1 hit from his giant energy ball

[Victor]

1) He using a combination
2)Defense doesn't stop elemental Heartless.
You're problem is you are underleveled. Don't even make an attempt on the Knight until you are lvl 99, but if you get your def to 99 which you probably will at you're rate and your strength up to 80 you shouldn't have a problem just give elixers to yourself and party members.