The Chimera, comprised of all the replacement boss monsters is more awesome than frightening, but the truly great part is the protagonist even managing to take the bastard down after wading through all of its components immediately before.
After spending the entire story being tormented by Red, finding out that he was responsible for the death of Melissa, and discovering that the cartridge is apparently an entire world of living creatures that are unendingly tortured by him, the narrator finally manages to kill the son of a bitch with the aid of Melissa and Acacius. "You'll pay for that", indeed.
In a meta sense, the sprite edits are amazing, and they were all custom-made soley for this story. The fact that the author did that in the first place is deserving of a medal.
The simple fact that Zachary beat a fighting game transformed into Platform Hell filled with SNK Bosses in a single sitting without getting a game-over (except for the first "RUN" sequence) is impressive in itself.
"I WOULD RATHER DIE THAN SERVE YOU."
The announcement that both Red and Solomon were going to be playable characters for Kaiju Combat.
Chapter 5 of Replay gives us Carl vs Warlock, the grey demon. Not only does Carl fight him off, he does so after having already fought two bosses in a row.
Zach shows some really badass qualities when he insults Red to his face. Note that this is after it became evident that Red knows of his precense.