Thread Rules: Post-Crisis, Pre-Flashpoint. In-character. No BFR - win by killing off or knocking out the opposition. Everyone has standard gear. No help from Dawg (not sure what help he'd be, but meh).
The Setup: The Slaughter League have come to beat up their former fourth member, Lobo, after he deserted the team in order to try to join the Justice Losers, cause, y'know, they could use the help. Superman and Martian Manhunter are the only others present in the Watchtower when Etrigan teleports in the SL, spoiling for a typical comicbook fight.
YOUR Job (the most important thread parameter): Come up with the most descriptive rundown of how this bad boy most likely plays out (in your opinion of course) with the comics supporting your stance.
As to the battle itself -- Slaughter League wins, comfortably. Really comfortably actually.
Etrigan can and has beaten both Superman, Lobo and Martian Manhunter.
Black Adam can and has beaten both Superman and Martian Manhunter.
Atrocitus can and has beaten Martian Manhunter, and took a good fight to Lobo.
The power difference between the SL and the last sons is pretty clear. The Last Sons do have two advantages here, however -- the first being that Lobo has the low-down on the Slaughter League and will have some clue on how to fight them. The second is the Watchtower, they have home turf and can turn it to their advantage. And honestly, I see both of those things coming into play, particularly the former -- I just don't think it will be enough. I don't think it'll be enough at all.
The Manhunter will scream endlessly in Etrigan's demonic flames.
Teth-Adam will chokeslam Superman into the side of the Watchtower like the little bitch he is and Shazam the f*** out of him like 20 times until he dies.
And Atrocitus will bathe the czarnian in Red plasma, an anti-regenerative substance so strong that it can prevent a Black Lantern from returning from the dead.
A bloody, gruesome, horrific fight -- and another win for the Slaughter League.