He's an ettin, and ettins come with Superior Two-Weapon Fighting. There doesn't seem to be any consensus regarding just how many attacks ettins are capable of in a round, taking the full-attack action. Here's a breakdown of the options for the 5-6 tier:
Option A: 2 mwk flails, +16/+16/+11
Option B: 2 mwk flails, +16/+16/+11/+11
I read that superior two-weapon fighting allows the ettin to essentially make a full-attack with each arm, thereby favoring option B. However, I would rather not get into an argument with my table tonight...
To top things off, the BBEG has both CLeave and Great Cleave. Cleave is a standard action...but if the ettin can make a full-attack with each arm, you've got:
Option A: Cleave at +16, additional attack at +16.
Option B: Cleave at +16 with one arm, Cleave at +16 with the other arm.
I'm really leaning towards option B...players in my neck of the woods tend to lean towards optimization, and even the season 4 scenarios haven't been posing too much of a challenge, and I like giving players a challenge. However, again, I would rather avoid an argument from whiny nerds.