The turtle boss had two phases.
In the first phase, it sent out waves of water crashing at the attackers, trying to weaken them and push them away.
At around half health, the turtle hid in its shell and began moving around the entire field, sliding across on top of the water. One needed to see in advance where they were going to be in order to dodge.
Jax had placed a Poison loop around the turtle's body right from the start, so he had an easy way of chipping into the boss's health. And from what Jax could tell, even changing phases did not help it get rid of that at all.
Now, all Jax really had to do was dodge around and wait for the turtle to die on its own. As the turtle was continuously being hurt by the poison, even with its head and limbs inside the shell, it could not stop taking damage.