It's just that there are tricks to resistance, much like there are tricks to any game. One of them is that there are certain invisible checkpoints or actions that cause the setting to change.
For example, when you push in on the enemy in some levels, teammates will pop out.
Also, sometimes if the dev doesn't want you to go a certain way, he will make all the enemies in that area extremely strong and accurate. The trick is to go around them or use the environment.
Kind of like when you went on the side of the road in old racing games you would take damage.
You really have to poke around to find the best way.
I myself spent a lot of time trying to attack one area and died like 9 times, so instead I went a different route and saw the save symbol show up and those guys I was trying to kill got hit by an air-raid and blew up.
