I think 1 flayer can easily be central to a fight along with 4 thralls (even better if its 3 thralls and 4 minion thralls). He can make it the most rivetign fight ever, depending on how some of his abilities actually end up working.
We don't know that a flayer thrall is effected by the mind balst, do we? Opens up a lot of possibilities if they aren't. Especially if the Mind Blast is rechargeable through brain eating!
Think of the flayer positioning his thralls before him, fighting the party. He dazes the party and not his thralls. The thralls are a big threat, being that they are right there smashing the party. They seem to be out shining the Flayer as the danger of the group, even though the daze effect is really screwing everything up (considering the potential daze effect, a Fighter's ability to control and defend are dropped to nothing if he is dazed since his defender capabilities seem mostly based on opportunity attacks). So now everyone is hurt, not just the defender. Everyone is getting brutalized. Thralls are running amok.
But wait! The one thrall that was hurt the most stumbled back to the Flayer! What's the flayer going to do? Heal it? That would be stupid! That wouldn't make any sense at all, that just-Wait! It just ate the thrall's brain! (No grappel or pinning necessary, no struggle could equal instant eats.)
And now the most weakened Thrall is dead (whoop-dee-doo - it was going in that direction anyway) and the flayer has another mind blast on deck to keep the party dazed and keep his other thralls wrecking house on them. This would work wonders if the Flayer has both regular monsters (for being the beef of the fight) and minions (for being chomp fodder). The Flayer becomes more of a threat the more thralls it has with it at any time. The flayer would not want to eat a solo monster's brain for a recharge (of course, I am assuming that that is a recharge...which is so cool of an idea, i really hope it is!) unless that creature was nearly dead anyway. Better to have weaklings, ones that won't overly weaken the flayer's position if he needed to kill them himself.
The party's first thought was "take down the thralls and get to the flayer," but it soon changes to "if we don't hammer that flayer down, these thralls are going to stomp us!" which is how I think the battle should go. The flayer should be a major threat without being the one doing the major damage, as it were. He should be controlling and planning and disabling, but not in the battle physically unless it is safe for him to do so.