To find this boss, follow my guide in
It is very easy to defeat the boss without taking out any Small Worms. You just need to aim for the Worm Tail and ignore all the worms. Just be aware that the game can lock on to a worm if it is closer than the tail when you start attacking.
If you kill the boss and do kill a worm, then you can use the map to travel back to this boss at any time to try again. You are not locked out if you miss it first time