Sea serpent

[abri]

My group consists of 8 players: 2 wood elves (Rg 6, Nec 3 F6), 1 half drow(W6), 3 humans (bard 5, Enchanter 7, cleric 7 [god of blood]), a half-fiend wood elf (F2/Cl1), and a half-fiend shade human (F1/Rg1/W3/Cl1) (very long story on this one).
Now 8 players are not really what the CR was designed to handle.
In the next scenario, they are supposed to destroy a secret Yuan-Ti temple: 6-8 low level Yuan-ti fighters, 2 L1 Y-T cleric and 1 L5-7 Yuan-Ti cleric.
Why the title?
Well as a dying act the main cleric is supposed to bring a giant sea serpent to attck the party as the temple on a clif start collapsing.
I don't really know how to design the sea serpent: I want him to challenge the party (They have a staff of life with 6 charge so I'm not worried about killing a few PC), but I want to make sure at least 2-3 of the player survives (after all they have been waiting for this fight for 8 sessions now).
Any advice on how to design the sea serpent?
For info, the team is nicely equiped (staff of mass haste, +1 wounding life stealing rapiere, +2 mithril full plate, +2 intelligent sword, amulet of neg energy burst and maybe 2-3 other items.) but with few items (most of the groups don't have magic weapons).

 

[The Sunderer]

Hi,

I would use an Elasmosaurus (MM Page 55) Advance it up to 15 HD and possibly push it up to Gargantuan size (which is beyond the rules - But who cares) and you have a nasty beastie!

 

[Carnifex]

Possibly use some sort of giant snake as a base, push up its hit dice, then slap a half-fiend template on it, or a half-water elemental template from the MotP on it. That should be a nasty surprise

 

[Lord Pendragon]

Look up the Boalisk in the Creature Catalog and advance it to Huge, and you should be in business.