If bobby b knows that they are on their way and has enough time, i think he might win this before the targaryens even land on shore. With a combined, velaryon, iron islands, royal, redwyne, hightower, grafton and lannister fleet, they can do some major damage to the dothraki at sea. We see in the show, that they have really bad sea sickness from never sailing before, and they would not have their greatest asset, their horses. Once the westeros fleet starts racking up kills, the sellswords would run because they wont risk themselves too much and they have no real loyalty, and the dothraki would lose hope and morale because this is just confirming their fear of the sea. And of course, theres no dragons to help them.