Here's the reality - 1236 needed to win, Trump currently has 67, Cruz 11, Rubio 10, and about 20 others among the other candidates.
There are 1351 bound delegates awarded proportionally with various thresholds of no more than 20%.
Total winner-take-all states are 832 bound delegates. Some of those states go statewide, some by congressional district, some require a threshold of 50% to take all the delegates, otherwise they are by congressional district.
The rest, 287 are unbound.
Which means Trump will have to take all the winner-take-all states, and average 22% of the vote of all the proportional contests, and grab the same percentage of unbound delegates just to take the nomination.
Which basically means he has to almost completely dominate everything from here. One major stumble will mean a brokered convention. Doable, but the bar is high.
Edit: this assumes a 3 candidate race all the way to convention. It changes dramatically if it goes to two candidates.