IT took a hot minute.
First I attacked and and as soon as he started running to the orb I try to beat him to it and destroy it as fast as I can lowering the amount of of time he is immune to damage ad regenerate. This gives me and good ol Dr Destoyer a chance to lay down some damage before he darts for another orb.
This is the only time I used super speed while in battle. it helped. The key is stay on his butt, kill the orbs, and go all out. Without the orbs, his regeneration is slow. Inside the orbs, if ya mess around, he can be back to full health each orb in no time.
None of my toons have or had any special gear just good old what ever dropped during regular missions and none were 40, the highest level one that soloed reistance was 37. The rest 35. Patience is key and some ability to do damage is also key. My toons are not pure DPS and none have crit chance, the ones that did resistance over 0.5% aka nill crit but in the range of tanks to somewhat brute equivalent. AOEs, help and when you can letting off the most powerful thing you can hit him with when ever you can and oh yeah again patience. 90% of the time dont have to worry about incoming damagebecause he usually ignore ya and focus on Dr. Destoyer.
A healer can help too because it keeps Doc. tip top and he attacks more the more health he has and does decent damage leaving ya to focu on killing the orbs that Shadow darts to.
I figured the easiest way is to kill as many orbs as you can while Shadow tangles with Doc. Because sometimes even if you destory an orb that Shadow is in, he sometimes simply darts to the next on and the next until he is either full again and or there isnt any more so kill them as many as ya can the first round that way you can pummel Shado and he have to wait until one regenerates which giveyou chance to get in some important extra hits. Then when one regenerates try to beat show to it, aka ignore shadow and kill theo rb as muc hbefore he gets to it. Remember when isnt inside an orb, he doesnt regenerate plus Doc is still zapping at him.