Go back to Orlen and catch a few sanders. Or, if you’re willing to try for a lower catch rate, go to the beginning of the desert and catch a few chopperbugs. Then distribute them evenly throughout your team.
Sanders and chopperbugs are arks with stun skin. Sanders’ stunskin has an 80% chance of activating upon death and chopperbugs’ stunskin has a 100% chance of activating upon death.
When stunskin activates, it will stop the bonus actions from the leviathans. Using this method, along with good hitters/tanks, you’ll beat it more easily. I used sanders and completed the quest on my third try,
The problem is pretty much just a bad assignment time. The three leviathans and the arkwing mission are two of the poor level-scaling missions, meaning unless you have a solid strategy (coughstuncough) you’ll probably be beaten when you are first able to accept it.