And how many times would a Rogue be able to Sneak Attack as a full round action? A surprise round is only a standard action, and any Rogue that Tumbles into the middle of an enemy formation will get tarred. Even if it is one big guy vs the entire party, if the Rogue walks up to him and sticks a dagger in him for 10d6 damage in one attack in Round 1, don't you think the guy is gonna turn around and tar him? This is a monster that is an appropriate CR to the party, which means he is at least equal to the Rogue in "levels".