Is it particularly important for Westig to be a fighter/ranger? I might drop those levels for four levels of Wizard or Druid. With an int/wis score of 12, he'd have access to second level spells (including resist energy, trololololo) and it wouldn't actually change his CR since casting classes are nonassociated class levels for giants, because giants specialize in bashing things to pieces.
If he was a druid, he could also have a pet squid and summon more aquatic nasties to harry the PCs as they try to take him down. Not to mention, he could heal the lethal damage he takes this way.
Even then, a CR 7 creature isn't really boss material against 6 ECL 6 PCs (which is a party level of 7.2, going off of the average CR guidelines. You're at just barely below average encounter material right now). I might throw in another level of the casting class (not enough to increase Westig's CR above 7, but enough to give him two second level spells assuming an ability score of 12. Hello, 10 resistance to fire and
acid which, if nothing else, will cost one of your casters a targeted Dispel Magic). Throw a couple crocodiles in the water for good measure. And for intimidation. They'll eat anyone that dares to cross the Mighty Westig, and raise the EL to an even 8, which is probably high enough considering the terrain disadvantage on the part of the PCs.
I might also include a few break-away planks on the bridge that'd cause the PC that stepped on them to fall into the pool if he failed a reflex save. Maybe attach the planks to a dinner bell so the crocs know it's feeding time.