Dig a looong tunnel, let gremlins multiply, maybe add a couple water traps in it. Hold 4, then hold 6, then 4...
I used to frequently use gremlin caves to jump a weapon proficiency level to ~9 in something like 5 minutes.
It's a perfectly valid technique but bear in mind that after around 3,5k gremlins killed, there will occasionally appear supergremlins, due to same monster type slain.
Those are pretty much extremely experienced gremlins that hit much like stuff in SMC would, so expect multiple attacks and frequent armor penetrations.
~200 HP and ~30 PV and you're good to go. Watch your HP though, especially after the 3k kills mark.
"Hell is empty and all the devils are here."