The invulnarability shield of this boss is the worst mechanic i have encountered in the game so far.
It all the way ruins my game.
5 times I have faced this boss and he kills me every time.
I am a spellcaster with havoc orb + turret and now I know that I CANNOT take expedetions until something is done about his invulnerability shield mechanic, as meeting this boss means a guaranteed death.
I just got to lvl 3 in an expedition with 0 deaths and as soon as i got the desert map I knew this would be my end, I got him to 50% hp by kiting him for 10 minutes and ultimately he regens to 90% hp and kills me 4 times.
Everytime I risk my life going in to remove his shield, it is removed and he puts it up again literally 3 seconds after, meaning in that time (where I alsso have to dodge his counterattack and the 4 railgun allies he constantly spawns) i can take around 2-3% of his max hp, and if I dont keep damaging him he will regen it.
I couldnt even get my Liver Mortis to taunt him.
I played with a melee friend and we could ill him almost like any other boss, in that case the mechanic was alright, but I'm sad to be stuck with the conclusion that I cannot play the expedetion part alone at all because when i meet this boss it means my expedition stops there and with no rewards.

It's not hard fighting him. You just can't blast projectiles and spells at him full force and expect to damage him. The trick is to get close so he literally lets his shield down, Than you attack. As soon as you break through his armor/forceshield or whatever you can really do some dmg when he is stunned. After he gets up again its the same process, get close, damage him and move away when he attacks.

Sadly I think this boss is a little buggy because I was about the fire my last shot that would kill him, but instead of winning the expedition I got teleported to town with the message my party was defeated which is kind of BS. I was at full health.