It will be easiest to get all of the Hunter Achievements on Easy difficulty. That way, the animals will do the least amount of damage and if you do end up dying you can respawn with no penalty. On Easy difficulty, the shark will take 16 stabs from a wooden spear to kill and 15 stabs from a metal spear. The metal spear also has a much longer durability.
There will always be a shark that follows your raft around as you play the game so it will be easy to kill a lot of them as you simply play the game. Sharks can be shot with a bow from your raft but that consumes a lot of arrows. They can also be stabbed with a spear while swimming underwater.
The best method for killing a shark is to swim at the shark head on and stab their face when they open their mouth to bite you. If you get the timing right you will interrupt their bite and they will swim through you without doing any damage. After this initial attack they will swim away for a bit after you stab them and you will be able to swim after them and get 1-2 stabs on their tail. If you have the flippers equipped, you will be able to chase after the shark more easily and get 3-4 stabs on their tail instead.