Before engaging with the larger party, identify your list of goals. Ensure you know, very clearly, what you want (with or without the larger party’s involvement).
Then, flesh out a few scenarios where you would be comfortable; almost a “list of compromises”: scenarios where you list of wants and needs (identified above) are arranged in various orders of importance and magnitudes. This makes you comfortably flexible when negotiating with that party - something that makes you look good, and at the same time you get to stick to what’s important to you.
NOTE: always be ready to walk away… Have a robust and reliable plan in your back pocket for use if things don’t go in a direction you are happy with. Having a “Plan X” is probably the most influential card you can play against a hand held by a “bigger partner”.