Heroes and Villains fight as always, but there are times where both sides must forge a truce and cooperate with each other to save their asses. Usually it's another villain who is worse than the villain at the moment, be it evilness or their goals.
Slade's Ensemble has a history of doing this with heroes team,
In Slade Strikes Back: While Slade has motvies of his own, he also wants to stop The Joker, So Slade and Bender decide to bring Joker down themselves.
In The V Team Island Advenure: He and The V Team team up with the heroes to combat Uka Uka and his forces
In The Great Time Travel Adventure: They do this again against Hunson Abadeer
In Legends of Darkness where their reforming is clear: Slade teams up with The B and Alpha Teams to stop Malefor, Darkseid and the newly ressurected Dr.Weil
The V Team has a history of this doing this with M.O.D.A.B once and The B Team twice which possibly was a cause of them becoming more anti heroic and the teams not fighting
The Z Force did this with The V Team and The B Team out of needing to surivive and needing to stop Uka Uka
Reaver and Former Members of Vilgax's allegiance did with The P Team, The B Team and The Alpha Team against Vilgax's allegiance
Professor Calamitous did this with The B Team and The V Team after being sold out by Iron Queen
Hunson Abadeer himself along his empire joined forces with Lizbeth and The V Crusaders against Discord and to a degree Sigma. This eventually extended to the hero teams of the story
Xigbar along with Xaldin and Shadow Queen joined forces with Tuxedo Lovelace. But then Lady Anastasia help Tuxedo escape in Birth of Miracle Elite. And a third time is the Grinch and Abominable Snowmonster teamed up with Terra to defeat Bizarro League. Shadow Queen's no longer allied with Tuxedo Lovelace and killed him.
Discord with Bender and Slade against Sari, Hazuma and Relius. Hunson Abadeer also did as well
Jesse with Bender, Slade, Discord, Castiel, Blue, Dr.Strange,Zick, Brock Samson, Kratos and Cruger against Blackgurumon, Loki, Albert Wesker, Dukat, Crowley and Pan. Hook, Anarky and Jareth do so as well.
Ares reforms and joins forces with the heroes against his father, Zeus, Johan Liebert, Sylar, Darkonda, Kingpin, Zorg, and Mecha Sonic.
Zeus joins forces with the heroes even Maka Albarn against Ra's Al Ghul, Shinnok, and Katherine.
Crowley forges an allaince with Bender and Slade to bring down Issac and The Syndicate as he got angry with them for trying to take over his domain.
Kuroto Dan, Gladion, Roxie, and Mandark decided for themselves to be recruited by the heroes to fight together against the united forces of Dark Lord Empire Federational Apocalypse-United Villains, Galactic Empire, Deus.Ex.Machina Industries, and Cobra Decepticon Empire.