They can be devastating. Run off, drink this, ash your cannon and run back. It's surprising how often this works. More than one enemy? Even better! Chances are one of them is facing the wrong direction :)
Isz Chalice has a room on third level filled with Brainsuckers, Sniper Kin shooting Arcane rays and Werewolves. Went through x5 Confederates trying to take that room, all of them died. Popped a few of these bad boys and Solid Snake killed all of them. Lifesaver.
These things are so damn useful in FRC Dungeons. You can't imagine how many times they safed me from Superior Beings guarding Treasures or Ritekeepers in these stupid Sens fortress rooms waiting behind the Axes spamming fireballs. Use these! Walk slow and use these! They are a Lifesaver in the Dungeons.
Best way to get a lot of these is to kill the Choir Hunter at Byrgenwerth and do the dupe glitch. Run back to Forbidden Woods to load/unload Byrgenwerth. Look up the exact locations you need to go on YT/Reddit. First 3 dropped will be where you killed them, the rest will be where the Hunter spawns. You can get the inventory max in like 10 min. Super useful item.
I used this and the gatling gun to beat that one shark guy guarding the Rakuyo hanging on the roof, made the bossfight semi easier, only problem is I didn't have quicksilver, and I was running a bloodtinge build