I actually feel like this problem might not be limited to Lord Erebus. There have definitely been several times where I thought that someone should get an assist, but they didn't. I can think of one recently where I threw mines under the feet of a demigod who died a couple seconds later, and the mines appeared to hit, but I didn't get an assist.
But it's tough to say whether these things are just me thinking that a player got a hit in, when it really didn't hit, or whether it's a real issue.
