I managed to beat it once (after about 4 tries) with a mostly random team of decently skilled players.
I like that it's challenging since normal was way too easy.
That being said, I don't like how these missions force you to do very specific things on onslaught. Let us get the rewards on either mode, and let people who want to challenge themselves play onslaught, either that, or remove some of the ridiculous restrictions (premade 5 being a major one).
You need to coordinate with your team, roam the map and take out important areas (such as when velkoz bunch up around)... clean them up to make the map easier.
When giant cho's spawn, have ur team kite and focus.
In the bullet hell, just survive til near the end then shield one player to flash forward and run to the end.
For the final boss, roam around and fight by pillars, clearing mobs before focusing boss..
Even with all of these points it's still super hard.
I think they should either make the mode easier, or better yet, remove some restrictions from the quests.