Even if you somehow manage to get through Chapter 5, which is possible, as soon as you get to Chapter VIII you once again have to use a walker but this time you face enemy walkers that have more health and do more damage and are impossible to avoid which makes finishing the mission impossible. We have no choice but to wait for a fix.

The devs have actually been aware of this bug for weeks now, ever since Onslaught first came out on the PTS but they didn't fix it, even though it's such a game breaking bug. They said they would send a fix this tuesday.

I was able to do it like you describe, problem is now i'm on Chapter VIII where you also use a walker but this time you face enemy walkers that have more health and do more damage and you can't avoid them which means you can't complete the mission. We have to wait for a patch to fix the issue. Here's a link for all the bugs and issues introduced in the 6.0 patch:

I just hope the devs are aware of this issue and fix it soon, otherwise we're gonna stay stuck and won't even be able to play the new expansion.

I've died like 10 times trying this. It's impossible to complete right now.