I've gotten about 1.5 playthroughs (1 on Hard, half on Impossible) in co-op (I'm playing as Carver/P2) and I'm working on my achievements. I've gotten a few of the weapon kill achievements so far and while working on getting "Blast Corps" I started using stasis (I pretty much never used it before) to double up on achievement progress. After I got "Blast Corps" I checked my achievement progress for stasis I only had about 5 stasis kills.
I also only have 15/500 limb dismemberments in 1.5 playthroughs. My go-to weapon thus far has been the military engine shotgun (I used it for probably 75% of my playtime so far). I see limbs fall off everytime I shotgun enemies, especially if I catch them in the shoulder/legs and have to follow up to kill them, but it doesn't seem like the shotgun or explosives add to my dismemberment count. Shotguns also don't seem to progress my human headshot achievement (with a decked-out military engine shotty human headshots are easy at medium range).
My brother on the other hand has 478 limbs dismembered and has used the line gun way more than I have (mostly through playthrough 1). We were both surprised we didn't get 500 limbs dismembered yet and I was pretty shocked when I saw 15/500 after all this time.
Am I required to dismember enemies with the cutter or linegun for achievement progress or is this some Carver bug?
Also, explosives (and possibly shotguns) don't seem to add to my stasis kills...am I limited to certain weapons for this achievement too? I would guess that maybe my shotgun/explosives 1-shot normal enemies so any limbs that fall off don't count because the enemy is already dead. Can someone confirm this?