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.

In general, having a huge crisis resolve quickly and very easily, relying on something not previously shown to the audience, is a deus ex machina, a god in the machine, and not a good thing. We have a number of [questions tagged with that][1]. The key to DEM prevention is foreshadowing, and this question is specifically about what to do when foreshadowing isn't available to you. Still, reading those questions may give you some more possibilities, or help you tone down that last-minute "oh yeah, I do have this magic wand in my pocket that I never mentioned before" that strains credibility.

There is another option which is to build up the item as the crisis builds. Like something is warm, or vibrating, or I-can't-explain-it-but-its-calling-me in the hero's backpack, or sewn into the lining of a coat. [Ideally you can call back here to something the audience did see when they got their backpack or picked up the stick for another reason.] Eventually our hero has to stop whatever crucial thing they're doing to pick the item up, causing some mini crisis like losing their weapon or getting separated from the group, and things get worse, and the item glows brighter or whatever, and then it ends up in their hand (since they don't have a weapon) and then -- BLAMMO! -- so the hero discovers this amazing power at the same time as the audience. But that is much harder to write.


  [1]: https://writing.stackexchange.com/questions/tagged/deus-ex-machina