They are in Fremmy Slayer Dungeon, so use fairy code A J R, goes right to entrance. Walk through for a bit, they are just after pyrefiends.
Mirror shield definitely needed.
Usually drop a small amount of coins, sometimes a few nats, water runes in sets of 75.
They regularly drop green charms, and sometimes red / gold.
Occasionally drop misc good items, such as tooth half of key & uncut diamond.
Drop lots of high lv herbs like Ranarr and Avantoe.
Don't hit very hard at all, but have average defence.