It's been possible to kill Diabolos before the second Omen since that content was current, but I wouldn't bet on a PUG being able to do it. Mostly everyone should have the HP to survive the first Omen now and he should generally be dead before the second.
Edit: Diabolos is stunnable, so a wily one should be able to keep Nightmares from going off and slowing DPS. It's still fun on the healer during/after that first Omen, though. I don't envy you guys.
Edit: Diabolos is stunnable, so a wily one should be able to keep Nightmares from going off and slowing DPS. It's still fun on the healer during/after that first Omen, though. I don't envy you guys.