A summary judgment would still need a suit be filed in civil court, and would need a plaintiff and a defendant. Cruz isn't going to sue himself and Trump doesn't have standing to sue so there will be no court decision.
But he does. Earlier ballot challenge cases got dismissed because 1) the plaintiffs could demonstrate a particularized injury, either because they were voters and stood in no different position relative to the population at large or 2) they were marginal candidates or potential electors for such candidate whose claim of injury was deemed speculative.
Trump is in a different position. He could commission a survey in a state that asks the common "Who is your first choice . . who is your second choice? questions. It would then be simple to count the number of instances where Cruz is first and Trump second and translate that into likely lost votes and lost delegates. With that in hand, I think Trump demonstrates the particularized injury the courts say is necessary for standing.
But this is in the end a political calculation, and given widespread belief the courts are very reluctant to interfere to such a degree in presidential elections, Trump will prefer to leave this issue hanging rather than risk losing in court.
Trump can sue in any state that has a ballot with both he and Cruz on it. He sues the secretary of state, asking the court to order Cruz name be removed from the ballot.
Prag is suing in this fashion. She's on the NH GOP primary ballot, so has standing. Has to exhaust her remedies before the election commission before filing in a court of law.