Could someone explain the joke?

The title of the image is "Dokyuukanko Daifukkatsu", or "Dreadnought Girl: Resurrection". From the Pixiv tags, I see this is a riff on the game title Dodonpachi Daifukkatsu. As you'll see from that link, this image parodies the promo art for the game.

Its funny because super dreadnaught girl the mangaka for denpa kyoushi also deals with a gigantic alien girl protecting Earth