Every member of the Justice League. This includes temporary/former members. Anything can go. (Hal getting every GL to assist, Bruce spends as much money as he wants, Superman can sundip for as long as he wants.) All characters are at their respective strongest and bloodlusted. Justice League has also found a way to negate aging on all members.
Round 1: Planetary Attack!
Location: Deep Space
Galactus and EVERY Herald.
Justice League gets a YEAR of prep.
Round 2: To the Underworld!
Location: Mephisto's dimension.
Mephisto, Nightmare, and an army of demons.
Justice League gets a DECADE of uninterrupted prep time.
Mephisto finds out a week before the JL attacks and begins prepping immediately.
Round 3: To the Dark Dimension!
Location: Dark Dimension
Dormammu and anything he conjures up.
Justice League gets a CENTURY of uninterrupted prep.
Round 4: Total Ragnorak With The Gods
Loki, Odin, Zeus, Hercules, Ares, Thor, and any other Marvel "God"
Justice League gets a 1000 years of prep.
The Gods get no prep.
Round 5: Absolute Suicide with Shuma Gorath
Location: Shuma Gorath's dimension.
Justice League gets 2000 years of prep, and everyone who they have fought (Lex Luthor, Darkseid, Spectre.) , including the people in this Gauntlet, join the fight.
Shuma Gorath gets 1000 years prep.
Can the Justice League prevail?