So long, and thanks for all the fish!
Its looking pretty vicious, its always a little tricky trying to assess creatures that act more than once a round. In this case its even trickier as you say you have 7 PCs, and thats a frightening amount of punishment they can dish out.
With 7 PCs I would be very tempted to use the old HP expression and give it 740 HPs, this might buy it one more round. I might also be prepared to give it another 250 HPs depending on how the encounter goes (giving it a total of 1000 HPs). Related to that is the minion summoning the Hydra does, 2 lots of 5 level 12 minions probably wont make that much of an impact, and you seem to have them appearing next to the Hydra.
I think I would probably want the minions appearing on or near the PCs back rank because that is where the Hydra has problems focusing attack power, while it has plenty of options within burst 3 or 4.
I might change New Arrivals to something like:
MINOR ACTIONS


Astral Hydra is subject to a critical hit)
Attack: Area burst 2 within 15, does not provoke opportunity attacks
(enemies in burst); +18 vs Fortitude
Hit: 10 damage and the target cannot teleport until the end of its next
turn
Effect: 5 Sahuagin Legionnaires appear in any free square within the burst
(see separate stat block) and act <INSERT INITIATIVE EXPRESSION
HERE>
On the Triggered Action front I might want to add something like:
TRIGGERED ACTION
Location Redirection (recharge

Trigger: A creature within 15 squares teleports
Attack (Immediate Interrupt): does not provoke opportunity attacks
(triggering creature); +18 vs Will
Hit: 2d6+10 damage and the Astral Hydra chooses the location that the
triggering creatures teleports to
Continuing the teleportation theme I might want an encounter power that would drag enemies into attack range
STANDARD ACTIONS

Attack: Burst 15 (enemies in burst); +18 vs Will
Hit: 2d8+9 damage and the target is teleported to any square within 3
squares of the Hydra
Miss: The target is pulled 2 squares and slowed until the end of their next
turn
Going back to the design of the creature I am a bit uncertain of the 2 action points, the Hydra has lots of options and a couple of multi attacks. Also with its powers having 3 chances of recharging on a 56 they should be available at some point each round. Personally I would hold onto the action points to use if the encounter was too easy for your PCs.
Actually looking at your Many Heads trait I can’t really see the need to have it make bonus saves against the dazed condition, action wise as it cannot move (and has no move actions) it really isn’t loosing that much by being dazed and the small advantage of it not being able to use minor actions or reactions is one I wouldn’t take away from your players. After all they are probably going to be knee deep in trouble so it seems a little cruel to take the dazed away from them.
Well those are my chaotic thoughts on your creature. It’s nice and dangerous, but I think against 7 PCs it needs a little more staying power or the encounter will be over very very quickly.