Aside from that, it is a simple monster to run (which is good) but I doubt it works as a BBEG for these reasons:

- Even with 3 turns it is vulnerable to dazes and stuns, because this effects apply to all heads. It may be better use the hydras rules (one dazed/stunned head per status effect used on it, player chooses head). Different heads and turns are supposed to be its strength so a rule covering these is needed.

- It lacks a ranged option, so unless the terrain greatly benefits it, ranged pcs are out of reach. As I see it, a capable defender can lock the monster for the entire combat.

- The defenses (particularly AC) are higher than expected. It seems as if you have applied old DMG 1 solo monsters rules.

- Damage seems to be ok for a brute using the new MM3 math. 15 cold damage at mid paragon seems low to me, a couple potions of resistance/utility power can reduce this to nearly nothing.