I can't see an obvious legal strategy offhand, which could invalidate the OGL easily in a court of law. Though that's not to say that somebody won't try, especially if one side has very deep pockets.
More likely the legal strategy will be for Hasbro/WotC to tie up Pathfinder or the retroclones in court, such that one side eventually runs out of money from legal fees and/or are willing to settle out of court. Even if in the end the OGL doesn't get invalidated in a court of law, Hasbro/WotC will have developed a bad reputation for being litigious against others using the OGL.