Even without their deadly cold attacks, the main reason they are so difficult is due to their high armor. They have a lot of HP and most weapons can only dent them. From the perspective of a firearm user, Aim Shot crits with the '44 Hammerer you get with 15 Persuasion does a good job against these critters and robots, but you can craft one with parts too. Make sure to stock up on high caliber armor piercing rounds as they are very effective here.
Still, since there are many within the building where the rescue mission is and if you want to save Newton, you have to kill all beetles.
In order to do that, what I always do, is just to shoot one, then close the door before others can get in. While in combat. The rest will investigate, but cannot enter. Wait a few turns until you can exit from combat, enter stealth, lure one in, close the door behind it and start to kill it. Repeat with another one by one, until all are dead.
In order to get proper gear, you can go through the Lurkers as their base (at least what's explorable at this stage) as they have some good loot. You have a chance to get a decent armor and also weapons once you reach the upper areas where there's a seeminlgy dead end with a small outpost behind a locked fence. There's almost always a guaranteed high-ish quality weapon there, and if you're lucky to get a sniper rifle, you can acutally one-shot the beetles with Aim Shot.
