Okay.
Two pronged strategy.
1. You have a white paper prepared demonstrating the nature of antifa, its history, its connections, its donors, its MO, etc. There is rampant information for such an effort, easily obtained and compiled given his resources; the culmination of which is, effectively, demonstration (NOT suggestion, DEMONSTRATION) of the FACT that "anti-fascist" is a codeword for "Communist", as it has been for literal decades now. The evidence of this is everywhere, and obvious.
2. You have a video piece put together utilizing that information which was employed in creation of the White paper, demonstrating the Communist/Marxist nature of these individuals, those who participate, lead and fund these organizations, and the organizations themselves. DEMONSTRATE beyond a shadow of a doubt the nature of these people, their ideals, and who supports them.
3. Produce a series of tweets containing the information drawn from the white paper, with photo, video and audio snippets to support each tweet. Have the tweets released in time with…
4. Obtain air time on network television - you're the president, you can do that pretty easily. Call it a state of emergency address and tell the networks to suck a dick. During a 20-30 minute address, walk the average American through this information, piece by piece, DEMONSTRATING to them the nature of these individuals/organizations.
That's honestly his best bet - come out hard, come out open, don't cuck, don't fucking mention 'racists' or 'white supremacists' or 'nazis' or ANY of that shit. You're there to talk about antifa, not who they oppose, and you don't have to talk about antifa long for their nature to be readily evident, especially when you've got the evidence to demonstrate it effectively right there and are going over it.
Trump cannot simply decry antifa, he has to justify why they deserve to be made a terrorist organization, beyond a shadow of a doubt.
Of course, there is a problem… Let's say he manages that, yes? Let's assume, as you say… Guess what? Now the media isn't really able to hound him on opposing 'anti-fascists', because we've established that 'anti-fascist' is codeword for 'communist'…. But now the media turns around and DEMANDS he ALSO declare these 'fascist' 'nazi' 'white supremacist' organizations as terrorists as well, while they likewise demonstrate these people ARE fascistic, Nazi-esque (if not outright), stand up for White interests (ie are 'White Supremacists'), etc… Like I said, dude kinda fucked himself pretty hard with that shit-show of a pre-emptive cuck coupled with his weak attempts at damage control after the fact.
The enemy saw an opening, they took it, and now Trump has to make a choice as to whether or not to step up and fight them, knowing that in doing so he's going to put himself in some rough spots going into the future (because cucking later by pushing Nazism and the like as equally bad as Communism will be just as bad as cucking now).