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.
that's the one that spawns the little wyrms, right?
My suggestion would be to keep using your lightning skills, and activate Armour of Storm once the adds come so you don't get overwhelmed. if you can, get at least one Writ of Fire off as an extra DoT.
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.