Tried soloing him 2x (120 Ranger) in Ruins of Lxanvom. Conned light blue. Both fights he stopped the fight and gated when he got below 20% HP. Could not cast on him or hit him. When he gated, he was immediately back to 100% health.
Mob do not summons but the 2 Guardian of Pestilence DO summons. I found out the hard way.. had to succor.. and bring more fire power. Grobbsby went down without any problems.. both mob double melee about 95 to 500+. Not too bad. They do not drop anything for me.