An example of the most powerful thing in the Diablo world that can do endgame damage and defeat the gods of the underworld is a man who wields axes of the finest steel, and wears the strongest armor a forge could make.
A decent estimate for a proper damage translation from Starcraft to Diablo would probably be 1 to 1000. A single marine could probably take a full account worth of top tier end game characters.
But diablo isn't just using regular axes, they're all magical. So the damage translation would remain what's in game.
So it would then be guys who deal like 30 damage each vs heroes who have like 5000hp and deal thousands of damage. They would have duped items, they would all have teleport, maxed immunity against all elements, their armor would be so high that no melee attacks would even hit them. That means zealots and pretty much the entire zerg race would be next to useless. The 3 Prime Evils could pretty much unleash their neverending armies and the worst part would be that for every Starcraft unit there is, Diablos minions would grow stronger.
Starcraft universe wouldn't ever be able to kill them.
It would be like the scene where kerrigan is by herself about to get killed by like 1000 zerg but then she would just teleport behind them, use a few meteor spells on their base, maybe summon like 1000 hydras and/or fire walls and it would be gg. Then a necro would pop in and revive all of them. A barb would use a level 30 shout making all of them beastly. And to top it all off, a paladin would show up giving everything thorns.
What would probably happen in the end is the starcraft guys would admit they can't destroy diablo's base (Sanctuary) so then they would just keep them pinned to that one planet and let them run out of resources. Since Sactuary doesn't have interplanetary technology they couldn't really counterattack.