Sarnemil doesn't have any special mitigations. Any kind of damage will do. It's specifically weak against Ancient Dwarf damage (like all dragons/wyrms) but your rune-stone won't be doing Ancient Dwarf damage.
While you burn at the stake, I dance with the flames. I take what you love and leave you in tears. I am relentless, unpredictable, and waiting for your last breath.
How are you having a hard time? It's not even a signature mob, it's a normal mob. Just slow and kite with lightning. Ignore the babies or drop a rock to distract them. This is pretty much the basics of playing a Rune-keeper. If you haven't figured this out yet then perhaps this class isn't for you.