As the picture shows,Ancient Fuelweaver could be easy to kill,thought skip second phase and had a nice meeting with Charlie.
He is like a target, and when there is no hatred for the life within the gateway, he just wants to go back and pull the key, and doesn't even bother to activate his skills until he dies.Sometimes just squeeze him into bone spurs and you're done (also worked for Toadstool).I guess this is no exception, since it was considered a Bug to avoid his boneshell nearby edge.Cause it skip all his abilities when AF's HP less than 10000.
Here's our advice:Whether or not there are things near the gateway,whatever the situation is now,activate his phase 2 immediately when his HP is below 10000.
The image down there is from Youtube.
Make sure that none around the gateway catches his attention when AF's HP is below 10000.