I wouldn't be surprised if it was by design. "Kill it with fire!" and "There was a spider." *House burns in the background*

There is something weird with the resistances in these mobs. They can take hundreds of minigun or magstorm bullets but they die easy to fire or to particle weapons.
The varuun rifles when maxed really kill these guys easy too, about 6 to 8 semiauto shots (around 1k damage each), same about to kill the big monster on niira and 2-3 shots most of the red mile monsters.
When you arrive at Londinion the 3 terrormorphs at the entrance took me about 25 shots max. I tried first the miniguns and the magstorms but they have little effect on these guys, they get into your face and start interrupting the reloads, that are also really long.
Its weird the game gives you a really good minigun for this purpose when i found out later after some testing that its one of the worst weapons to fight them. Miniguns and magshear or magstorms with piercing bullets are devastating against humans, but not so good against those monster bullet sponge minibosses.

It's funny, but it explains a lot. I ran into one randomly in a cave, and was using a laser rifle that did fire damage, and I was like "why are these things supposed to be the bane of existence". Now I understand.