So I was shooting beehives in Balenos forest when I came across Dastard Bheg, the field boss version. I am a seasonal Dosa with AP 241 DP 320. I spent the next hour trying to kill him solo and was not able to significantly damage him in any way. Most of the time IMMUNE kept popping up, but his health was going down very slowly. It was down to about 15-20% when he suddenly despawned.

I didn't see anyone else hitting him so I assume he is on a timer.

Anyone know why I was doing so little damage to him? Is there a minimum AP required to do enough dmg to kill him? I went and attacked a Rift III version of Bheg to see how it compared and without any potions/food buffs, with the same equipment I was able to kill him in 90 secs. So is there a threshold AP I need to meet before I can kill the field boss version?

Field bosses are meant to be done with more than 1 person, usually 20-30+. I'm assuming you were on a season channel, and that's why no one else showed up. If you want to do bheg, go to a regular channel where he's up and follow the boss train.