A list of crucial items needed can be found further down the page. (ccar.net)
As soon as we find out one such crucial item is available, we'll give you a heads up, regardless of the fact that the timing of the content seems absurd. (themomedit.com)
Between each wave, there will be a short period where you can reinforce your defenses or go on side missions to obtain crucial items. (metalgearinformer.com)