You nitwit! Heroes are Extinct!! (2003)

Blurb:
General Cassiel has come from light years away with the intention of conquering Earth, and more importantly, meeting Earth’s warriors in battles for a showdown of epic anime proportions! But what will he do when he discovers that no heroes have come out to meet him in battle? Will he claim an easy victory; or, do Earth’s heroes just need a little help getting out the door?

Source: www.anisearch.com/manga/1171