Complex sentence: Trump has repeatedly asserted that U.S. and Iraqi military leaders should used "the element of surprise" to attack Mosul rather than announcing plans beforehand and said that U.S. military planners were "a group of losers" for not doing so.
Split into simpler sentences: Trump has repeatedly asserted that U.S. and Iraqi military leaders should have used "the element of surprise" to attack Mosul rather than announcing plans beforehand. He also said that U.S. military planners were "a group of losers" for not doing so.