well i have a tauren in my lineup so it makes it a bit easier to catch a fleeing druid. i just mount up, run at him till i'm on top of him, frost shock, purge, then war stomp. At this point I begin casting 3x lightning bolt, and hopefully I can finish casting that + chain lightning before he gets away out of LOS. The combined LB + CL should take him low enough that a shock will kill him. Continue purging and chasing, he should hopefully be dead by then.

Staggered earthbind totems around the pilllar he's "loving" also helps to catch him.