Depends on how ‘hard science’ you want to be. There cannot be an ‘energy creature’ in real physics. ‘Energy’, treated as an object is simply a bastardization of mass/energy conservation by the general public and New Age woo. You can not simply have Energy; to make energy into an object is to store in the form of some mass, say petroleum.
So when you say convert mass into energy, it isn’t turning into another ‘object’, it is being turned into massless particles (photons). That massless photon exists only as long as it takes to interact with a particle with mass, a detector, your eye, or Russell’s teapot, at which point it might turned into another photon, or matter (binding other molecules and atom, like a plant making sugar).
If an energy create were to exist, it would have to be able to localise itself well enough to be a threat. Which in turn would have to mean it would have to redirect its energy sometimes. Which would require a physical medium or at least mass. Which then would mean it is not an energy creature anymore or could not exist outside say the horizon of a blackhole.
If you want to bend the laws of physics for a good story...
If the create can exist for some reason, one way to defeat it would to be expose it the vacuum of space where its photons could not be stopped. They would whiz away at the speed of light into the infinite beyond. It would become so cold/diffuse that it would not be a threat.
For a more technobable solution, perhaps it could be defeated by a (another scientifically inaccurate) Blackbody Device. Here they can Ghostbuster the creature into a prison where its energy cannot escape. How to get it in the prison would need some hand-waving too.
Another would be the good old, black hole as others have described, though a little cliche.
Photons operate within space-time field that is affected by gravity. Any gravity weapon would affect the creature as well as creatures with mass. Perhaps a gravity weapon would create a micro-black hole that consumes the creature's photons which would immediately evaporate and the creature suffers damage.
Remember it is your monster, it may come into existence by some strange physics and can be defeated by as such.