First of all, make sure the creature can actually succeed at its primary method of fighting, i.e. using grapple attacks against multiple foes. Give it the following two abilities:
Superior Grappler (Ex): When Zargon grapples an opponent, it is not considered grappled itself. It can grapple as many foes as it has tentacles at the same time. It can attack a grappled creature with its natural attacks (except for the limbs with which it grapples, of course) without penalty. If it hits an already grappled opponent with another natural attack, it can make another grapple check as a free action to immediately pin the opponent.
Reactive Grappler (Ex): Whenever an opponent within Zargon's reach misses a melee attack against it, Zargon can attempt a grapple check against that opponent as an immediate action. If it succeeds, Zargon and the opponent are now grappling, and Zargon deals tentacle damage to the opponent.
Also, for some kind of ranged option, how about:
Fling Opponent (Ex): In place of a tentacle attack, Zargon can hurl a pinned opponent at another opponent whom it currently does not grapple. This ends the grapple with the pinned opponent. Both the pinned opponent and the target of this attack take 2d6+16 points of damage from the impact and end up prone. The target of this attack can attempt a Reflex save (DC 26 [Str based DC]) for half damage and to avoid being knocked prone. The attack has a range increment of 10' (starting at Zargon's normal reach of 15').
Finally, a little bit of added defense:
Hostage Shield (Ex): Whenever Zargon is pinning an opponent, as an immediate action, it can negate a single successful attack (melee or ranged) against itself and redirect it at the pinned opponent, who suffers the full normal effect of the attack. Only one pinned opponent can be used as a hostage shield.
Note that while these abilities sound rather nasty taken together, immediate actions are 1/round only, and Freedom of Movement completely shuts down most of Zargon's special moves. Hmmm... maybe consider some ability that counters or partially counters Freedom of Movement?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.