invasion onslaught vel'koz is bullshit

The fact that they can 100-0 you (in fact probably at least 300-0 you) on their own from across the map is the BS part. Not every champion can even hit them from across the map, and I don't think every ult available would kill them on their own (global ults I mean)

The game mode is a total disaster right now, community is calling for a hotfix but It doesn't look like Riot will be answering that call.

Not really, you need to be coordinated and take them out ASAP. Using Ezreal/Lux and Jing helps a lot. Use poppy to push other enemes off of your team while you handle vel koz. It's hard, yes, but not impossible.