system
Given the following post, respond as if you're commenting on Reddit:
human
[Terminator] The films show Arnold terminators (T-800/850s) to be some of the best and resilient ones. How can the humans fight armies of them if just one is so damn powerful?

So the first few films cement the fact that arnold's model are powerful and in many cases, BETTER than some of the more later models developed by Skynet, due to its ability to learn.

How come we're supposed to believe the resistance is mowing down armies of these terminators if just one (Arnold) was a one-man army in the films?

I'm just finding it a bit hard to believe that in the future, hundreds of T-850s are disposed like foot soldiers when in the films just one of them is the damn protagonist.
gpt
Two main reasons, technology and knowing your enemy. 

In the future, we have the future guns. You can clearly see in the flashbacks of the war the humans aren't using regular assault rifle. The terminators are known enemies so every soldier knows Armour piercing and explosive rounds must be used, that is the norm, not some regular shotguns you find in the back of police car.

Also in the past (present), we never see the T-800/850s to go up against the super hard hitting stuff the military has. It's always some pistol, shotgun, and some kind of police force. Remember a regular pipebomb in the first movie was enough to split one in half; imagine what an automatic grenade launcher can do.