Also kind surprising that you're just getting that achievement now? Would've assumed you got that ages ago considering how easy it is to get once you can reach that area.
I think the effect probably didn't trigger any of the first two or three times i attempted to use it (attempts probably only made during my first runs),
and then never used it again, partially because it's a melee weapon, but mostly because of the vague description.
My understanding of it was something like this :
"It definitely won't work on all enemies, that would completely break the game. Therefore, some robots are immune to it. Which ones? Probably those with coring immunity."
(With this understanding of it, the weapon lost most of its use cases.)
"I don't think it's worth it to spend two slots for an unreliable melee weapon that drains tons of energy and can only kill enemies i can already kill quickly anyway, especially when i always play ranged builds, so i don't see the point to it."
Then, when i started doing flight builds :
"Still not worth it, the Broadsword is much more reliable at killing enemies, costs less energy, and so on."
Eventually, i heard that it could kill a certain large prototype robot that does have coring immunity, but that didn't clarify what it could actually kill.
I think i might have tried to use the Core Stripper against that robot, but i'm not sure. It definitely didn't succeed, though.
And i never saw the point anyway, since my builds couldn't benefit from that robot's parts.
It's only recently that which robots were immune to it was clarified (i think that change is from Beta 14?),
but even then, the immunity to the Core Stripper is written about only in the "Dismemberment immunity" explanation,
instead of within the part description itself (like is done for some other parts, like the Immobilizer and the Field Lobotomy Kit).
I think the description would be a lot clearer if it had "doesn't work on robots with dismemberment immunity" at the end of it.
The only reason i actually got the achievement is because i saw that someone had gotten it rather easily, and because i saw the two bugfixes about the Obliterator (which i haven't gotten yet),
which allowed me to deduce how to get the achievement. I was then able to get it the very next run.
So yeah, that's why it's the 220th achievement i got.