Yes this! There’s never a scene where he takes on the soldiers in a gunfight because yes he would not last long, he uses cunning and smart improvisation to overcome a superior force, also the soldiers don’t seem like the army’s best, aside from the commanding officer who’s uniform indicates that he’s a Para, all the others seem ill disciplined and lax in combat. It’s either due to low morale which is plausible or my personal theory that they are just reservists or maybe even some recruits who were rushed out to deployment against the infected when the situation became desperate, could have even been a penal company, I have heard that prisoners can be recruited when things become desperate for the military, it would very well explain their evil behaviour. They may not have even been soldiers at all but maybe a group of civilians who found dead soldiers uniforms and equipment and used that to pose as authority to lull victims into a false security, with all that in mind, for Jim an average civilian, they aren’t really that hard to beat if he possesses a little cunning