FTG user won't wait for the user to cover himself with his Susano'o and then cover it with Amaterasu. He'll be dead by the time he covers himself with Susano'o. By the way, FTG isn't an offensive attack, it depends on the user whether he want to attack his opponent or not. FTG is just a means to reach the target quickly. So it's not a coward's tool. The combination of the attack and the FTG is not a coward's jutsu.
OT : As I said, FTG isn't an offensive jutsu. It's a space time ninjutsu. So if the user,
for example Minato, uses FTG on Sasuke covered with susano'o and amaterasu, can easily stomp with a giant rasengan and a clone on Sasuke. Now the question will be that rasengan can only increase the flames. But the rasengan will also damage the susano'o and indirectly the flames can also harm Sasuke if not controlled quickly. So Amaterasu is a double edged sword. And Minato can use a clone of his instead of attacking by himself against a wind-fire jutsu.
for example, Tobirama, Tobirama's clone needs a jet of water to break through the susano'o without considering the flames as I said it's a double edge sword.