Read It Back

[Gildarts]

Since this was kind of a small one I just added a few tweaks using <>

Rep wrote:

Forcing himself to recover after the battle against the Heartless, Repliku realized a few facts. One<,>without the ability to open Corridors of Darkness there was no way he could possibly get up to that floating island in the sky, <this fact alone> made him feel insignificant. Two, if any attackers came down from that island< , he (instead of he, use a pronoun like Clone or keyblade user? Just to keep the zest of it zazzy)> was for lack of a better term<,> quite thoroughly screwed. After all, arrows or other projectiles from that high up would be hard for him to predict or block. <In addition to that Also (keeps things short and less wordy)> without the ability to return fire in any way, shape or form, Repliku was in trouble should someone <attack from that altitude>.

This part above "... should someone attack from that altitude" was another hint of repetition sourced from your previous sentence about any attackers coming down. Maybe change the wordage or imply instead of a person, some magical catastrophe coming down. What if the entire island fell?

To make matters worse, however, he didn't know much about the Omniverse in general< ,  and>to top things off, summoning things with Omnilium was still not a skill that was in his wheelhouse. His attempt to summon the book that he had wanted had been at best laughable and that had <which (a little less wordy)> made things impossible for him in the long run. 

The Heartless situation here also required that he wield something he didn't have, presently.

I wouldn't keep the above comma in and I would consider merging the two sentences between "presently" and "he'd have to get.."

He'd have to get some information on how one could summon a Keyblade, or become worthy of such a weapon. It was <after all> the only way he'd survive his next encounter with the Heartless, if any of them were stronger than the Shadows he had encountered.

Sliced out "after all" because it helps the flow of the sentence down the line!

Shuddering as he imagined being devoured by the Heartless, Repliku made up his mind on something. While he wasn't his Template, he could <+however> play the role of a hero and fight the abominations as best he could. Of course<,> in order to do that, he was going to need to learn more, and not just physically. He'd need to regain the magic potential that he had lost, including Dark Firaga, and Corridors of Darkness.