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?