He already has the aquatic subtype as he's a Scrag.

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.

If you're looking at JUST a scrag, then this fight is going to be weaksauce. He's going to get completely overwhelmed by the PC's overwhelmingly favorable number of actions or get one shotted by a spellcaster's will save spell. As-is, this is barely a speedbump.

Throw in three or four Giant Crocodiles that the ranger has trained. That should even things out.

The Scrag is the ranger. He's a figher 1/ranger 1.

Didn't notice that. Whoops. I still think him ducking down into the water would be an interesting tactic. I like he crocodile idea. Maybe have the bridge collapse so that they have to kill the crocs before the boss.

Right. Which is why I was saying, since he's a Ranger, it's entirely within the realm of possibility that he's trained some animal assistance. Sorry I wasn't clear there.

I'd rate this encounter as weaksauce, for a few reasons:

1. EL vs party level: A scrag fighter 1/ranger 1 would make a decent boss for 2-3 PCs of sixth level. Against half a dozen or more, though, he should be tough enough to actually accomplish something before he gets crushed. You want to throw a higher level encounter at them if this is going to be more than a miniboss.

2. Action economy and party size: Wetsig gets one action to their six or seven. Even if he wasn't just 1 CR above their ECL, he'd be at a disadvantage.

3. Combat style: Sure he's a regenerating meatshield, and

If they've taken some feats from Stormwrack to make fire spells work in water, this fight would be easier. The water's surface normally blocks Line of Effect for fire spells, which is one saving grace of the original idea.

the party can't really hurt him without acid... but he has to get close to hurt them. Even if he's hurling javelins, the party's mage only has to drop a Ray of Exhaustion for a no-save win, or use Deep Slumber to go for the scrag's weak save.

Now, how do we fix this? Here's a few ideas:

1. EL: You're probably aiming for an EL 10 encounter here, based on party size (bosses are usually 2-3 CR above a 4-man party, if they're solo). Rather than beef Wetsig up to where he will almost automatically destroy one or two PCs, I'd add additional creatures. Crocodiles work, but you should also consider adding something that can fly, as a means of challenging those pesky mages. There's plenty of options, but a bat swarm springs to mind as something that would be appropriate.

2. Action economy: Solved by adding more creatures. Wetsig can focus on getting the PCs into the water, while his croc (or crocs) can do their thing. One giant crocodile and two bat swarms should do the trick, and the swarms force the casters to choose between blasting the bats or lobbing acid at the troll.

3. Combat style: By staying out of sight in the deep, murky water, Wetsig can set the battlefield up to his advantage. With the resident bats and his tamed croc, he can be more than a punching bag, without having too much personal power (a failed Will save will still ruin him, for instance).

4. Terrain: Don't forget that Fireball and similar area spells also do damage to objects (minus hardness). Tossing a Fireball at the pilings could bring the bridge down... with the party still on it!

I edited the OP with some new information.

Still no real skill monkey, so traps are out of the question.

Druid at level 6 will make the water less of an obstacle, so I would recommend throwing some beasties in there to take on a wildshaped druid at their level, especially if said druid has "natural spell."

Warmage is fairly easy to counter, just take a look at what spells the Warmage has available. One thing to consider is if that Warmage didn't bother with any acid spells, to keep that scrag under water as long as possible, since above the water the scrag will be incredibly vulnerable to a warmage's fire attacks.

... Still needs moar crocodiles.