The Mothza Supreme is a huge Bugsnack that lives in the Sizzling Sands area of Bugsnax. To catch it, you will need to complete various missions for Cromdo and he will eventually ask for your help in catching it. Cromdo will lead you to the pyramid in Sizzling Sands, then he will open the top of the pyramid. A plate will pop up that is blocked by some form of crust.
To break this open, head down to the sarcophagus in front of the statue. On the side of the statue, you will be able to pull the large pizza slicer down using your Snak Grappler.
A Bombino will fall out out of the broken sarcophagus, and you need to bring this to the top of the pyramid and place it on the plate using R2. It will explode, clearing the plate, and then you can place your launchpad on it. The launchpad will slot into place perfectly, and the Mothza Supreme will come in closer to the pyramid.
The Mothza Supreme will stop over each of the four statues, causing small sandstorms to appear. Use your Sauce Slicer and some hot sauce to lure the nearby Bombinos into the storms. This will cause the fire to go out, and you can pick them up and bring them to the top of the pyramid. Remember, you will need to wait for the catch counter to run out before you can pick them up instead of catching them.
Go to the top of the pyramid and place the Bombino on the launchpad, then fire it at the Mothza Supreme. Now, run down to the Mothza Supreme and use that statue to hit it with the pizza slicer.
You will need to do this twice, then the fight will change slightly. With only two statues left, the Mothza Supreme will spin between them, causing huge sandstorms when it comes to a stop at one of the statues. Grab a Bombino and ride the largest wind tunnel as high as you can, then throw the Bombino at the Mothza Supreme. You just need to hit it once this way, then hit it with the giant pizza slicer once more to finish the fight. Grab the Mothza Supreme after it has been cut down to size and feed it to Cromdo.