UFO Robo Grendizer

Same creator. Grendizer made a cameo and the demons are implied to be aliens too and use spaceships and UFO. 

Both TV series are adapations of stories of Go Nagai. The hero transforms into (Hiroshi) or controls (Fleed) a giant robot. With this robot, he fights one or several giant monsters per episode. Usually, the episodes follow similar patterns, with the final boss fight in the last third. There are occasional episodes not as formulaic as the others which develop the backstories of the characters. Over time, both the hero and his allies and their enemies gain additional powers. You will see lots of colorful monster designs, and explosions. Even if the monsters only last one episode, they all have individual names. In exceptional cases, they get a backstory. It can be quite repetive, and I would not recommend watching more than 5 episodes in a row. But for occasional brainless watching of "monster of the week"-stories, I think these titles are quite OK.  read more 

Both of them are exceptional super robot series of the 70's that every fan should watch!