Felt I should post as most of the solo'able threads I read mislead me to believe that I, a 68 Necro, could simply aggro this guy and walk right out of cave into an open field and kite him... easy as pie.
First, the minute you aggro from tunnel all his guards about him come charging plus 2 guards and a giant spider that does an AE snare pop in the tunnel, thwarting any attempt at a quick escape... and also any chance to FD... the spider constantly does the AE as long as you are within range.
The easiest solution, I found, was I brought 2 other Necro friends, made a sharp left into his chamber, and made camp along the furthest wall. The other 2 Necro FD, I snared the guard on the left hand corner (as you're facing them), Rallius runs over without bringing any of his other guards, no giant spider AE, and only the two guards that spawn in the tunnel come and really aren't a threat as you FD once Rallius smacks you once. He loves to camp corpses, so he won't move for a long while. Let your snare wear off, all aggro'd guards walk away, enemies forget you, stand up and snare Rallius, sic pets, other two Necro get up, nuke him to death with Lifetaps in about 2 minutes (pathing is horrible, he'd hit me, I'd walk a little left, he'd walk backward, then walk forward again to try and hit me again... I'd have 3 taps off by the time he hit me again) Piece of cake and a fun kill to boot. Looted head, SP, and gloves, gated.
Course, modify for solo Necro adventure... but getting to him will be a pain. The spiders throughout the tunnel CH... so it's nice to have a lot of damage and stuns coming from multiple pets. At any rate, have fun!

Arachnid Antennae
Cursed Armplates
Decomposing Wristband
Ethereal Parchment
Gloves of Flowing Ooze
Oozing Quarterstaff
Oozing Skull
Pox Ridden Carapace
Rattican's Tooth
Spectral Parchment