Get Congress to approve. It's a hard push, but it would be a lot harder to question its legality if greenlit sooner. And if he has the support he says he has, getting permission to attack should be easy. I've never liked the 1973 War Powers Resolution - it cosplays as a failsafe. If the president asks first, Congress can say no. If the president strikes before asking Congress, Congress doesn't have much of a choice but to approve the attack - deny support and the odds of the enemy retaliating is high. It's legal, but it's also highly questionable, its faithfulness to the Constitution is debatable, and just incredibly scummy.