So we just fought a nest of giant mantis shrimp and I was wondering about their stunning attack. So it doesn't say that the stun doesn't stack nor can we find anywhere where it says stun doesn't stack. I was wondering if anyone knew if it says somewhere that stun from this creature doesn't stack. If it does then I hate these creatures with a passion.
Have not looked at giant mantis shrimp specifically, but:
Unless they say otherwise, most durations will overlap, not stack. So if you get stunned for 2 rounds then immediately get hit with a 3 round stun, you will be stunned for 3 rounds, not 5.
Similarly, if you get stunned for 3 rounds and 2 rounds pass (leaving you with 1 more round of being stunned) and you get hit with a 2 round stun, you will be stunned for 2 rounds.
For reference: Giant Mantis Shrimp.
Sonic Burst (Ex) A giant mantis shrimp attacks with such extreme speed that its pincers issue a concussive blast of light and force when they snap closed. Each time a giant mantis shrimp makes a pincer attack, whether it hits or misses, each creature in a 10-foot-radius burst centered on the attack’s target takes 1d8 points of sonic damage and must succeed at a DC 20 Reflex save or be stunned for 1 round. A creature that succeeds at this save is immune to the stun effect from additional sonic bursts from that giant mantis shrimp until the vermin’s next turn, but it still takes sonic damage from additional sonic bursts. The giant mantis shrimp is immune to this effect. The save DC is Constitution-based.
Speedy Pincers (Ex) A giant mantis shrimp can make iterative attacks with one pincer.
So basically, they attack 3 times and trigger a 10' radius DC 20 Reflex or stunned 1 round regardless of hitting. Once you save though, you are immune to the stun for 1 round. CR 9, but I would strongly recommend hitting them with Slow...
The silence spell cast on yourself would give total immunity to the sonic damage and stun effect. With a bit of planning the encounter should be straightforward. Mantis shrimp are unintelligent instinct hunters and so should not engage in sophisticated combat tactics.
Dangle some bait to coax them out into the open (eg high ac, silence shielded character, who can turn invisible to escape) and then blast from a distance.