Darkseid invades the MU earth.

Darkseid invades the Marvel earth with the ALE and a vast army of his para-demons, no bfr.

1st scenario, the MU heroes are unprepared.

2. They are well aware of DS coming attack giving the heroes top scientific minds and strategist time to prep for the attack, what happens next?

Bane tries to stop the invasion and gets fisted by Gladiator

The same thing that happens in Thanos invades DC, the good guys win due to shenanigans like they always do or they bring out the big guns like squirrel girl.

War Machine kills him