Does Shandien have his Second Wind left? He could use it to heal up and bolsters his defenses a bit before moving away and provoking an OA from the big skellie. I think he doesn't have to do anything to maintain his Mark on it, but he could always use his minor to refresh the mark. To get the skeleton at least 3 squares away from Thurnan, he'll have to go somewhere down into the southwest corner of the map, say G4 or so. After the big skeleton chases him off to down there (assuming it follows, but if it's marked I'd assume it would mindlessly go after Shandien with no one to direct it otherwise being a mindless skellie) we blow it up.
Meelblen, can we use minor actions to make Knowledge Religion/Arcana checks to figure out how to keep these skeletons from respawning, and deactivate the field of necrotic energy around Thurnan? I would assume since it's just for information it would be a minor action, rather than a standard since we're not doing anything (unlike actually using Religion/Arcana to do something, like disable a trap).