If you really are releasing your story in pieces, and at the high point of piece #7, you want your character to use an item that they must have obtained back in piece #3, but you didn't mention in piece #3, don't interrupt the action to flashback and explain where the item came from. Make it a mystery. Describe what's happening (a glow coming from the character's hand, or the opponent just freezing in place for no apparent reason) and all the consequences with little exposition at all. Then, when the excitement is all over, have another character (representing the audience) ask "what was THAT?".

At this point your character can explain why this is in fact not a total surprise you just pulled out of nowhere because you didn't have a good way to end the battle. Ideally, this explanation would refer only to things you showed the audience in earlier pieces (say an aside in narration about 3 days of boring training we would never need, or a dumb heavy pack they all have to carry for no reason) that now gain significance as the character explains them. But if you're really stuck, it might be enough that the other character already knew about them.

> What was THAT?
>
> A thermo-thauma bridge, of course. [Cheerful smile.] Mediated through a pencil because that's all I had. You didn't skip the third day of thermo training, did you? That's when it really got useful.
>
>The first two days were so boring, I went to the health centre and got a skip note. 
>
>Well, I bet you're glad I didn't!

It might take you a while, but look back through your earlier pieces for some "blah blah blah" moments like training you don't describe, reading you don't describe, annoying packing or supply lists, and see if you can pretend you meant those all along to be foreshadowings of this moment.