Did ash really team up with gym leaders and battle a giant pokemon or am I having a fever dream?

I feel like I remember a couple of episodes that I can't find anymore amd it's driving me crazy. Did ash and his pokemon really team up with the power players in the anime to fight a larger threat? I honestly can't find them on bulbapedia.

Like a scene with them all on a bridge or dam or something and the big baddie coming across the desert at them?