A rookie with a single promo II-ish EMP grenade could one shot those. Actually, rookie can throw 3-4, per turn, so, you know. 3-4 one-shots. And that's with captured suits intact! Imagine the possibilities...
The notion that a crappy rifle is "OP" or even relatively good is completely hilarious.
I mean, I see that your point is mainly about low-tech and stuff. But 150 accuracy soldiers can solo entire missions with proper weapons, so it's kinda moot.
What kind of armor would a single soldier need to solo an entire HQ or a base? Or a battleship?
If the weapons are powerful yet compact alien designs, then the point is moot indeed. However, if
the designs are familiar kinetic rifles, then we have a logical contradiction with reality. That means that there is an issue with the computer model of that reality. In practice, even a marksman rifle could not destroy a tank in real combat. In-game, such possibility should not exist as well. Also, it from the purely game standpoint, it is desirable to nudge some evolution in weapons into the domain of energy weapons, and other advanced designs.
The rifle in question is not an anti-materiel weapon. It could not conceivably defeat that level of armor.
I would also like to mention that EMP grenades are very plausible and fitting weapons for destroying such armored targets. Those types of weapons in reality would have a similar effect, possibly even on a modern tank (albeit, not WW-2 era one). These grenades model the reality correctly and make sense in-game.