There are 5 of them that spawn at ~78. The two wearing the black shirt (clones of each other) can be killed. The other three are wearing the same type of shirt and hatch into Strelats upon death: yellow, white, and green.
At ~97, another one will spawn wearing a purple shirt. This one can be killed.
Edit: Any kind of head-crushing melee will prevent the mutation. Also, if something explodes too close to these guys, they will not mutate either. Keep this in mind if you plan to kill them at 130+ because a bunch of Telo-Explo (white exploding dudes) will spawn at this time. You can kill them with explosions, but make sure it's not too close to them.
I manage to get to 90+ combo chain (but didn't manage to kill 150). However, I didn't encounter any strelat since I followed what wowitsyugi said. I managed to headcrush those machine gun dudes, so no strelat mutation for me.
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