OK, some changes made to the shambler. I bumped the CR up to 12, mostly HP increases and additional legendary resistance. I'm trying something suggested by Unearthed Arcana : Encounter Building, that is to make a really deadly encounter, treat the PCs as if there were 50% more of them. (I.e. 6 6th-levels = budgeted XP of 8400 = CR12 foe) I don't think the added HP will really make the fight grindy, right?

@Miniman 119 from 113. But added 1 L.Resistance to make it 2. And removed magic resistance (I feel like I keep giving my bosses magic resistance, which is sorta unfounded)

Oh wait, cr12 bumps up the legendary resistance tier... recalculating...

So thats recomputed to (119+60(l.res))x1.25 = 223 effective hp

With 15 AC +2 for improved saves = 17 ac.

@daze413 119 can't possibly be a grind.