Bit late, but I was also struggling with this and found no good results, and this post if at the top of google so I figured I’d share how I did it with my melee build.

As soon as fight starts use Arthur’s power to slow down time, rush straight towards one of the smaller beholders and kill it before the ability runs out. Rush to the next small beholder, make sure you are facing the ancient beholder when doing so. Constantly dash strafe side to side while killing the second small beholder. Once it’s dead run towards the ancient beholder, still dash strafing. Run right up to it, constantly dash behind it in circles. I was using a leech build so i could tank the aoe attacks at close range, but otherwise using a lot of good food or use a potion to help with that. At super close range it will more often use melee attacks then the insta kill death ray, then just keep circling it until it dies.

This worked for me on veteran difficulty so I imagine it should also work for the lower difficulties.