I found the Lyndworm to be too easy. I beat him in my first try during the 3rd wave. And not because I'm such a good gamer... but because the recipe to kill him was obvious, and pretty much works the same way as dozens of other games across multiple genres. Survive a wave of enemies/obstacles, wait for the invulnerability to pass, then kill. Rinse and repeat as necessary.
Also, this is not the final boss. There are 2 other boss fights after this, depending on the choice you make. I actually found the real final boss to be much, much harder. It took me a few tries.