Okay this is a bit wierd. I fired up a quick game and soon had the immortal in question join. Putting him in the rear of the army didn't work. Sometimes he gravitates over to the larger group of archers anyway (because the archers move forward as the enemy rout). Bodyguards indeed doesn't work (not the first time I've given bad advice
).
Putting him in front of the army or on the flank with cast, cast, attack, attack, cast & spells made him disease the least amount of troops but he still blundered into some of the units.
The moral lesson is this: He's a complete jerk. The description doesn't lie the least. He's an immortal grumpy old monster that wants to make your life miserable. Stick him with immune units like the undead. Atleast they won't mind his endless ramblings about "wussy good-fer-nuffin gods these days".
