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As you delve into the X-Labs, aside from bumping into more of the fucking horrible Lopers, you'll also see the Prototype Super Soldiers.
These guys aren't trivial, they'll tank lots of damage, and are well armed. Unlike the Lopers, they're also loyal and in control, and won't just senselessly gun down everything in sight, rather they will focus on blowing you into smithereens, or hunting down and destroying escaped Lopers (which they usually accomplish well). Unlike the Lopers, you will also see these a few times after the X-Labs.
They have pretty good aim with their machine guns, and also sometimes have rocket munitions, they will also try to avoid grenades and explosives, on top of completely shrugging off the flamethrower, so you really can't be as cheap with them as you can with Lopers.