You can demurrer and/or file a plea in abatement to their lack of standing, but that might not be a good long term strategy if they can just resurrect their LLC or re-file on behalf of their principals.
Your best bet is to hire a lawyer.Litigation isn't a DIY job and cant; be learned on the fly.
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Your post does not indicate the current status of the litigation. Have you filed an Answer yet? If not, you can file a Demurrer on the grounds of lack of capacity to sue. If yes, you can file a Motion for Judgment on the Pleadings for the same basic reason.
This is not a permanent fix because the court can stay the litigation and allow the LLC sufficient time to revive itself if it so elects.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.