BigCo promises you the original source code + game assets.
Instead of promising Alpha/Beta/Release from the signing of the contract, promise X/Y/Z business days from delivery of those critical assets.
(Make sure you have a timeout clause in there too, "Assets will be delivered by BigCo within 60 days of the signing of this contract or we will not develop that title and BigCo will pay a penalty of $X")
[edit to add] Be careful to define what "assets" mean to you. Assume what you write will be read by a lowest-bidder consultant, with active disincentives against showing initiative, and who can barely read. (Not stupid though)