^RAMA mentioned Robot Jox in the first post. It's mecha vs. mecha, not mecha vs. kaiju, so it doesn't fit the category under discussion here.
I rewatched Robot Jox just recently. It's an interesting film, with a script by SF great Joe Haldeman (I gather the director dumbed it down a lot from what Haldeman wanted, but there's still some intelligence there).
Joe Haldeman is fairly pleased with the movie as it turned out. It had a great music score as well.
Robot Wars, and another film were loose sequels to Robot Jox, but even these robot vs robot movies are uncommon Except in even lower grade B movies.