Rate of diffusion of a gas across a membrane is not simply a function of molecule size but also of apperature size. Therefore the slow passing of O2 and CO2 could not be a problem while the more rapid breath weapon/gaseous form simply is blocked.
Or "You have been slowly seeping through the gaps in the force cage for an hour now and suddenly your head and chest solidify on one side and your legs and belly on the other. That was a nice creative attempt but you've left a bit of mess. What do the rest of you do?"
Also, the ELH implies that a wall of force is not perfect give that epic escape artists can eventually slip between the gaps. And then there is the cube of force.
Or "You have been slowly seeping through the gaps in the force cage for an hour now and suddenly your head and chest solidify on one side and your legs and belly on the other. That was a nice creative attempt but you've left a bit of mess. What do the rest of you do?"
Also, the ELH implies that a wall of force is not perfect give that epic escape artists can eventually slip between the gaps. And then there is the cube of force.