I asked in another thread but think it got buried in the posts content. But I am having a real problem with the 2nd boss on heroic grimrail depot...

In the other thread somebody mentioned a 'sweet spot' to stand but after 2 hours trying to find it- well I need help. The problem I have is when he gets in the cannon. At this point I'm not sure if I'm just suppose to hide my team, dps the ads, and use the cannon? Or am I suppose to find this sweet spot and dps him out?