He doesn't go there. Trump is actually an exceptionally disciplined campaigner (no one has noticed this, shhh). Trump has his chosen lines of attack:
•Rubio's a no-show
•he "defrauded" Florida by ditching them instead of representing them
•he's weak on illegal immigration with the Gang of 8
•he sweats profusely
•guzzles water
•he's a lightweight ("Little Marco")
Three policy hits and three lighter mocking slams. Trump riffs on those specific hits, rat-tat-tat, every chance he gets, but won't go beyond them, he has his game plan and sticks to it.
And his game plan is effective. Devastating, even, as evidenced by the fact that "Little Marco" is about to be squashed like a bug on his home turf today.
You are correct. I have been impressed with Trump’s discipline. He is a master of timing with the appropriate drive by weapon of the moment.