
If you're just looking to beat Onslaught, the team comp above makes it pretty easy. I haven't managed to do it without a Poppy and Raka in the group though.
A bit harder, but a good MF can replace Jinx, and still have a fairly easy time. Also Ahri can be replaced by Syndra and still not have to rough a go of it.
The heals and tankiness from poppy and raka are really what make it easy. As well, charming or stunning the large vel'koz helps a lot. Almost beat it with Janna, but couldn't beat the final boss. Although, that was more an issue of our team not using obstacles well enough.
Overall it seems to really come down to people being able to play the given champ at a semi decent level. Sometimes you'll get a few people that don't have a clue how to use their champs and it just doesn't work out.
Really do enjoy this game mode though, reminds me a bit of WoW!
Edit: On the Vel Koz level, raka heal poppy and stand behind poppy while lulu shields. You'll have to move with her, but she can soak most of the damage. Once you get near the end flash out of the circle and end it. Main priority here is to keep Poppy and Raka alive.