The Anti-Monitor(COIE), Imperiex Prime, Mageddon, Parallax, Ion, and Monarch and his army invade the 616 Universe of Marvel. They head to Earth to eradicate it from existence. Earth has no prior knowledge of these beings...and the DC crew catches Earth with just hours to prepare before entering the atmosphere.
Asgard and Olympus can help Earth out in the beginning. If Earth can survive for thirty-six hours....then any cosmic/abstract help throughout the Universe...short of the IG and LT...can come to Earth's aide.
No UN for Reed Richards....Galactus has it...and he can't arrive for thirty-six hours...if Earth survives that long.
The effects of Mageddon will be felt on Earth as soon as the DC crew enters the 616...but, Earth will have no idea what is causing the chaos until the DC team is just outside Earth's atmosphere.
I'm really going with the more recent Earth...so, WWH can be in this fight. Although....classic Earth characters can be used also. It doesn't matter who they are...as long as they are below Universal in power...so, MJJ can't be used. As long as the character resides on Earth full-time..or close to full-time...then the character can be used.
After 36 hours....then Galactus and his Heralds can participate...along with any other UNIVERSAL cosmics/abstracts. No multi-versal being can be used. The UN is the only multi-versal weapon that can be used in this fight....but only after 36 hours...and only if Galactus is able to use it. You know...barring he isn't KO'd or something...before he can get the chance to use it.
Not much they can actually do any person on the team can solo Tiamut well almost it looks really bad for Marvel but Iceman can hold them off for an hour or so.
Remember that Asgard and Olympus are here to help. This would all include the most powerful characters from each. Odin...even Rune King Thor, Zeus, Mangog, Surter...he can even use his Twilight Sword or Casket of Ancient Winters...all Frost Giants