I have actually killed a level 20 alpha mosa, starting the fight at mid-health, without having scuba gear and only with the help of an ichtyosaur.
So basically, some few facts you need to know about mosas in general is that they have a really wide turning circle, meaning it is very hard for them to do a 180° turn. Also, I think you all know this but they all stay in the depths, they can’t go any higher than a certain depth. Alpha mosas actually regenerate their health quite slowly.
It wasn’t actually that hard: I just needed to constantly hit it on its back with my ichtyosaur. When it will try to bite you, it will have to do a 180° turn, so it won’t be able to fight I back as long as you’re hitting it on its back. Whatever happens, try to stay away from its mouth: you can easily outrun it because ichtyosaurs are really fast so be careful because you might come too close from the mouth. It surprisingly doesn’t do as much health as I thought it would do, but don’t underestimate this fishy boi’s jaws power. If you do everything correctly, you should take all the 60k hp in around 10 mins.
If you’re not a high level player like me, playing in solo, and so you don’t have scuba equipment, don’t forget to breathe: it may seem obvious but drowning is actually the thing that dealt me the most damage, and I actually had to use backups around 3 times.
Also be careful of the night: it is actually hard to see where is it’s head and where is its tail during nighttime so don’t be dumb like me and don’t accidentally stumble on its jaws. By the way it’s freezing cold in the depths during the night so don’t die of hypothermia.
I was playing on mobile, so nothing actually attacks ichtyosaurs, so the only thing that will try to kill you is the alpha mosa.
The most important thing is to stay concentrated on both your oxygen bar and the position of its head.
Well, this is how I actually defeated my first alpha mosa, I hope this was useful ^^
Imagine like being the ultimate underwater apex predator and a weird looking fish riding a dolphin swims towards you and bites you for 10 minutes straight until you die
A good way of looking for these things (mobile): sweep the border of the map and the ocean floor, using an ichthy to keep everything away. Once you find one, park your ichthy somewhere, then swim up to the depth limit of the mosa (you might have to test where it is though, be careful, mosas can swim up FAST). Being off the ichthy will catch its attention. Swimming up to the depth limit will cause it to Aggro off of you, so go back and forth with going to its depth limit, attracting it, and swimming back up to fill it with tranqs. The basic idea is attract, swim up, fill with tranqs, repeat. Hope this helps!