I mean, he fought well vs someone who has a genius intelligence rating
Plague Demon can also jump out

why would he just stand there and let knuckles sink him?
If he can’t jump, he can use the shockwaves from Plague boost explosions to launch out like a rocket ship
Plus he’s used to fighting in underground/cramped places like the new monster association base, so it’s not like being stuck or sinking down is a back breaker unless he gets really trapped.
Without a resistance, I assume it’s just as effective against him as it would be against anybody, so the radiation stacked up by mutually explosions is gonna effect knuckles extremely fast, if he also gets infected by Plague Demon after being irradiated it he’ll probably just succumb to that on the spot (no extra willpower to help this), and then he wouldn’t be able to dig or burn and then would just perish
He can push away 7-A Sora at least a little bit (even before advancing to next key), shouldn’t have much problem shooting knuckles away like a cannonball, especially with knuckles LS being so low next to his own and Sora’s Class G. The air shot attack would be devastating.
Plague also has regen and the punch would be slowed down by Plague Boost Aura, in addition to the speed amp Plague should be able to just dodge it if that happens