You can't sue someone unless you can prove damages. There's no way to fight this without breaking the rule first. I'm not a lawyer though so maybe there's an exception to this rule.
NAL but you can sue without breaking contract if you're challenging the contract's legality. Epic also is generally challenging Apple's use of their position to 1) not allow other app stores, and 2) take 30% of each sale (while other processors take 3%), which would be enough damages to sue over. Epic breaking contract is entirely a PR move.
They have proved that, already. They could have reverted the direct payment system, kept only Apple IAP, and then sued even for damage recovery of 30% since the beginning of the lawsuit (or even the beginning of Fortnite on iOS). Apple has clearly written even in legal documents of the lawsuit that reverting the direct payment change would have been enough to keep operating Fortnite on iOS fully featured.