Mermaids have it rough. Being a marine species, they lack the means and quite possibly the inclination (as mermaids, like people, get used to what they have) to create fire and work metal, and therefore will be stuck in a relatively primitive tech level.
As a result, their weaponry is going to be bone, coral, and whatever metal they can gain from trade with sailors or salvage from shipwrecks. A sharpened rib, tied to a piece of driftwood with seaweed, for example.
This poses an obvious problem, as they need to deal with predators. Not just sharks and giant octopi, but other monstrosities as well-things like Snappers, mutant octopi, dire wolf-fish, giant sea snakes (which are big enough to swallow a mermaid whole), giant spider crabs (ditto), and saltwater scorpions (big, fast, and tough enough to prey on mermaids, basically a giant marine water scorpion).
Now, mermaids are about as fast and manueverable as dolphins, they have human ingenuity and intelligence, they live in schools of up to twenty 'maids, and they're highly altruistic-a mermaid would die for the good of the school if it was necessary.
The only problem is their lack of options-yes, intelligence has allowed us to conquer just about any predator here on land, but they lack advanced technology, which I think will impair these mermaids to do the same. Even if they trade with sailors, those sailors are restricted to medieval-grade technology, and metal doesn't do well in saltwater, so I'm not sure how they will adapt to deal with predators.
Things That May Affect Answer:
Mermaids are viable trifold chimeras created through magic, so given the right conditions, fish or whale genes can be expressed or mutated to give mermaids natural weaponry, like fangs, lionfish fins, spines, or a narwhal horn. In other words, natural selection gives mermaids a little leeway here-if they can survive. You see, these genes are dormant from the beginning of mermaids.
As an all-female species, mermaids survive thanks to sailors. This will likely influence the natural weaponry they develop, or how they adapt to predators. In fact, if it turns out they can't survive in the ocean, chances are mermaids will board ship and take their chances in captivity, but that's a last resort.
An answer is viable if it includes a trait (such as a natural weapon), item (which mermaids could feasibly attain or craft), or social behavior/course of action mermaids could use to deal with predators. General answers are suggested, but specific answers, if possible with the information above, would be preferred. If more information is required, let me know and I will add ASAP.