Based upon what I see, you are importing the body temperature. Then you really are giving Mechanical nothing to solve as you are telling it what the solution is.

Think back to your heat transfer class. What type of heat transfer do we have here? And when solving convection problems, what was the one term we were constantly having to find using correlations? That is the quantity you want to be taking from Fluent and mapping onto the surface of your Mechanical model. Then you can set up this transient analysis with an initial temperature of 300° in the body and then the software will calculate the temperature field in the body.

Keep in mind with the heat transfer coefficient you also need to define the free stream air temperature.