I noticed this a while ago but keep forgetting to add: Kostchtchie has mythic enlarge person as a spell like ability, presumably meant so he can make himself colossal before he goes into melee. However, as he actually an outsider and not a humanoid or giant it wont work on him? Was this a mistake? He could, of course, still cast it on other giants working for him but using on himself just seems so much more his style. BTW, why is there no Enlarge/reduce monster to go with charm and hold monster?
Regarding presumptions, does this creature show up in any published modules? I see a reference to Descent into Midnight on one of the wikis. If the creature shows up there, it ought to have tactics listed.
Edit: Well, that won't help. The module in question predates mythic rules by quite a bit.