Goo canisters - go up the stairs, open a goo canister, a friendly blob should spawn. Leave it there, and eventually the turret should shoot at it. Repeat at least once. How well/quickly/safely this works has varied a good bit* with changes in vision, etc, so be careful. (I haven’t update in a few months, so I’m not sure the current state of things, here.)
When you go up the stairs again, count the shells (easiest with V) - if there are 100 or more, the turret is out of ammo. I usually try to leave the turret as long as possible, as it’s the best source of light.
Another way is to simply layer all the clothes you possibly can, go upstairs during the day, so the turret shoots at you at maximum range (while you’re still on the stairs) and take a round of fire… if it misses enough/you get lucky enough on hits, take another round, etc… when you get hurt any significant amount, go back downstairs to heal.
The theme here is to get it to run out of ammo (at 8 rounds per burst, it will generally run out in 13 bursts), as that’s almost always easier (in my experience) AND more useful AND more reliable than destroying it.
*At one point, you could go upstairs at night, and the turret wouldn’t see you until you got a little closer than the stairs, so you could build a piece of furniture to block line of sight (if you had the skills/tools) and push it up closer, which made this work even better, since you could drop the blob closer.
