hitting the blad ein any fashion would be like Roy trying to hold it as it is kicked by a horse...
sure it may give DK a major owie, but it could disarm Roy or break his wrist.
Hitting the flat of the blade if he holds it stiffly would...
Hitting the blade is gonna be equivilent to hitting the blade of a chainsaw with wood. Except worse, cause flesh yields better.
DK is gonna be hitting him with something he's never had to deal with strength-wise when it comes to blocking, even if DK would get hurt.
He can only block the flat.
also, can Roy of all the swordsmen swing the greatsword multiple times before DK could punch?
*sigh*
You seem to have a fundamentally wrong mental picture here, something I tried to correct by saying "jab", but evidential it didn't work, so let me lay it out.
You've got 3 basic classifications of blows (many fall in between), names are made up since there's no official (aka, everyone calls it something different, or refers to the individual blow and it's properties).
1. Jabs, these depend on being close enough to attack with essentially a wrist flick and minor movement, often done in conjunction with with an advance. Generally weak, but with a weapon their equivilents can do significant damage to unamored foes. They're also faster then reaction time.
2. Medium: requires more time and commitment, basically requires more momentum but does more damage, generally more range and is about at reaction time. Can do signifigant damage to armored foes.
3. Heavy: your killing strikes, requires full commitment from entire body and wind-up as well, significantly slower then reaction time, used as a punish if the opponent made a mistake generally.
Jabs will be multiples per punch, DK is already in range and he can't keep roy out because he lacks a way to safely control the space. Roy can therefore freely poke with them. He's unarmored, so a single hit to the most exposed area (the head) means death, this means that DK needs to sacrifice maneuverability for defense and stand on his hind legs (which he's really not smart enough to do in the first place).
DK can't do the same because Roy is too far, so moving close enough for a jab will get him hit during attack preparation (fencing concept).
Then comes the next problem, because of the spacing, Roy can do any of these strikes safely since DK will not be able to punish. And then there's the fact that Roy can make minor adjustments (in the wrist mostly but also arms) to account for dodging in the heavy strikes mid-strike.
Basically DK needs to precisely predict anything Roy does in order to do... well anything to him.