I think one of the easiest ways to get black pearls is to get a mosa keep leveling health and damage about 15K hp and 300-400 damage. Then find and destroy the alpha mosa. (you might want a saddle with good armor) Bring a Megalodon or other creature to the squid, once he is grabbed very quickly swim to the squid and feed the pearls you got from the alpha. That is how I have tamed my two squids and it works very well. (I play mobile so I don’t know in it will work on pc or xbox)
1: The best place to encounter tuso is 90-91 lat and cover all longitude from red ob to the end of herb Island.
2: Don't solo search, make a party and cover full bottom of the map.
3: Don't search in shallow water and surface, go for mid and deep sea.
4: They need to grab your tame then only can they be tamed.(Take Carbonemys with health and speed pumped)
5: They take 50 black pearls per serving.
6: They only agro in deep sea.
7: Last and the most important, the server must be brutal or their chance of spawning is not that much. Put server to brutal and for one day atleast go killingspree in ocean to open spawn spots for tuso.
You need two people, one for feeding the black pearls and the other to feed the turtle that is in the grasp of the Tusoteuthis, with sweet vegetable cake for the health and to feed it stimulates berries as well