"...the likes of which have never..."
Someone fed The Donald this construction, and he has made it his own, now. It is so unlike anything else he ever says that it's clear it didn't come from his addled brain.
Personally, I'm guessing he got the wording from Steve Bannon, who often uses sort of outdated, but correct, English in his writings. Bannon probably fed him the "fire and fury" thing, taken from Harry Truman's speech, and added the "...the likes of which have never been seen..." which probably also came from Bannon.
Now that he has the phrase, he's taken to using it in other ways. The Donald has learned a new phrase, and will repeat it again and again, until he wears it out completely. It's become a stock part of the regular nonsense he emits from his mouth.
However, this is not the first time Trump has used the phrase, courtesy of Mr. Bannon. In fact, he used it in his inauguration speech:
A historic movement the likes of which the world has never seen, he said in his Inauguration Address on Friday. Never? Ever? The Bolsheviks might quibble. So might the veterans of the fight in this country for civil rights. You know, people like John Lewis.
https://www.nytimes.com/interactive/projects/cp/opinion/presidential-inauguration-2017/trumps-mystifying-movement