Mazinkaiser Tai Shin Getter Robo

Crossover one-shot between Mazinkaiser and Shin Getter Robo. It was included in Immortal Super Robot Encyclopedia (不滅のスーパーロボット大全).
The manga begins with the Mazinger Z and Great Mazinger fighting against an alien machine after the Diana A and Venus A have been taken down.
(Source: Mazinger Wikia)