Run and hope they fight each other? Seriously though it's essentially mathematically impossible, barring some really elaborate eewok-trap type stuff. They cannot hit except on a 20, do pathetic damage even if they somehow do get through its armor, and can't call their sharks to help out. Even if all of the sahuagin charge the same crab, and even if one of rolled a 20 every single round, (very unlikely since two would die every time the crab had a turn), you're still looking at on average less than 70 damage. There's just no way.
Why is this necessary?
But the answer is probably Alchemist's fire.
or assists.
several can assist the attack or other skill attempt of another...
the same way humans can kill a mammoth. trick it into going over a cliff, or falling into a trap. shipwrecker crabs have no int score at all, why do you think that sahuagin, who have MUCH better statistics than an average human, wouldn't be able to pull it off with a bit of preparation?
Looking at old message boards on sahuagin, sahuagin have a swim speed of 60. They could safely take out the giant crab with their heavy crossbows at the second range increment, but they would use a lot of bolts. The range penalties would be -2 for second increment of range plus -2 for every 5' of water. So only natural 20 hits would work. As long as they had plenty of ammunition and no objective to defend they would all live.
Yeah, I think basic hit-and-run style tactics are their best bet. But you specifically say "On land." Hm.
Honestly, I think the "Ewok style traps" idea makes most sense. The Sahuagin set up traps on land, then lure the crabs in. They drop boulders and tree logs on them, that sort of thing.
Let's consider the chief weaknesses of the crab:
1. No DR or spell resistance.
2. No form of regeneration.
3. No intelligence score.
4. No climb speed.
5. It can only kill 3 creatures a round at most (2 claw attacks + Attack of Opportunity.)
6. Despite their size, their land speed is only 30.
What I would do if I were the sahuagin:
First, try to make the crabs fight each other. The best way I can think of is making some sort of catapult and loading carcasses inside, then hurling them near the crabs from a long ways away. If the crabs are presented with limited food, maybe they'll fight each other, thus reducing your opponents to one (or to two injured crabs).
Alternatively, wait for them to split off from each other.
Whatever your methods, your goal is to only fight one crab at once. When you do so, find a really, really steep cliff on which to fight. There are two ways to use the cliff to your advantage:
A) Have someone lure it near the bottom. Then just fire tons of arrows/bolts/stones at it because 1 in 20 will hit and do damage. Make sure the cliff is tall enough to not end you.
B) Same thing, except you drop boulders on it from above.
The reason I do not suggest the "force it off a cliff" tactic is because where mammoths are intelligent herbivores, this crab is a HUUUUUGE person-eating monster. It also moves just as fast as people on land. It can kill 3 of your 20 people in 1 round. Why would you want to try and scare it off a cliff? Needlessly risky.
Bombarding it from the safety of the cliff while it's scuttling on the ground below is much safer.
However, there IS an idea similar to the mammoth tactic that WOULD work: dig a freaking pit. Seriously, if you dig a pit deep enough, you could fill it with spikes and stuff at the bottom and then lure the crab into the pit. Make sure this is DEEP. You can't have the crab getting out, and remember that this thing scuttles ships for a living. This would have to be some sort of long, arduous project, or you'd need to find a big enough hole already existing and cover it with some sort of giant fake ground? Yeah, the shoot-it-from-a-cliff trick is way easier.
Another option? Tame some local wildlife that have a movespeed greater than 30 (50+ would be optimal, but 40 is most likely). Ride the wildlife as mounts. Since you've only got a 1/20 chance of hitting anyway, you don't need mounted archery; just piss off the crab and fire at it while your mount runs away as fast as it possibly can. 1/20 shots will hit. Use shortbows or light crossbows or javelins.