You are really looking for something like a flame-thrower, or a large area of napalm, or starting a forest fire...
... or you're using one of the extremely fiery explosives that the punisher is using.
And there are two options for the shield - either it is permeable or it is not. If it is not permeable, then sound wave vibrations will be transferred from the air molecules to the shield, the shield will start vibrating, and this will be transferred to the air molecules on the other side of the shield. this will be a very inefficient process, so sound will be greatly reduced. Since heat is also kinetic energy of molecules, the kinetic energy would have to be transferred through the shield, once again a very slow and inefficient process. However, these are all minor points: the major point is that if the shield is not permeable, air molecules will not be able to pass through, and he will suffocate if he keeps it up continuously.
If the shield is permable, then the question is how permeable it is. As the permeability increases, the airflow between the inside and the outside increases, thus allowing duncan a greater air intake. However, it would also allow greater transfer of soundwaves, shockwaves and heat. With the larger shields, the flow of air would not be important, so the permeability can be turned down very low. However, with duncan's shield, he's going to need a very large airflow to allow for exertion in combat, and this would make him susceptible to shockwaves and heat.