I think it sounds like a bad idea, but I'll give it a chance for a few issues at least.
ripcurl's forum posts
The Marvel team wins because of (the seriously underrated) Deathbird.
She trained Gladiator, and it's been stated numerous times that she is much stronger than the average Shi'ar.
She's powerful enough to go toe-to-toe with Carol Danvers.
First fight Marciano wins easily.
Marciano weighed between 178 and 192 lbs. during his pro career. Bruce's giving up 40 lbs. MINIMUM.
Second fight Bruce Lee wins. With the ability to use kicks and grappling (Bruce Lee trained with Judo legend Gene LeBell, who has trained several professional mma fighters) he has a huge advantage.
They don't have to "damage" adamantium.
Darkseid could melt it with the omega beams, tear his body apart at the molecular level (matter manipulation/transmutation) or possibly just drain his energy like Galactus did in Secret War.
And no, he's nowhere near a match for them physically. Ultron''s in the 75-ton range, while Thanos, Darkseid and Despero are all well above class 100, planetbusters.