I do have the Splinter, otherwise I probably wouldn't have embarked upon this strategy.
I left many weaker enemies behind in the first four islands suspecting/hoping I would get the pain skull again on Nameless -- and I did for the 3rd game in a row, is this item guaranteed? -- so once I purchased that I went back to the first four islands to do some business, namely pain skull, moro's darkness, gladiator sword and spamming stone skin for medal all in one.
It got rather tedious eventually (grinding = not much fun), especially with the skull which progresses
really sluggishly, but thankfully that is now behind me.
My first attempt at Hagni using either black knights + vampires or just knights proved unsuccessful, so I gained a few more levels and got level 2 onslaught and level 2 adrenaline, which made all the difference.
As for gaining rage, I agree that it's slow, or non-existent rather, when the stone-skinned knights get attacked, but you guys aren't getting enough rage from all of his retaliations? Isn't that one of the benefits of the Splinter? (P.S.: Also have Anger 2)