How about you just whisper bad ideas to it until it gets so depressed about it that it commits suicide.

If it's gonna eat a whole bunch of people anymore, how's about we feed it the people we don't like first?

Attack his weakness. The ultimate weapon is a great way of defeating a monster.

In Blue Seed, they defeated a giant jellyfish monster by injecting it with the liquid absorbing solids that are found in dipers and feminine napkins. Turned the insides of the monster into a solid rock, which killed it.

It's a Deus Ex Machina ending, because your players will have to 'happen' upon some sort of killing device before they find it, but as long as it isn't a crutch...