I made progress this time: I got through its shield, but then from the description it's supposed to stay down if hit by an EMP but it absolutely did not stay down. So the whole 3M hitpoint shield reset!

My strat: Lots of shields

I figured it out. After a many hours long battle where there was basically no progress, I tried sending in a melee fighter. That's what did it. You can easily stun lock him. I then brought in another pawn to double it. I think they could have done it all on their own after a very long time. I think 1 could have even done it. But I realized that I could move all my shooters in under its shield and take him out much faster.

In the end it felt cheap. I don't think it's possible to win with ranged weapons. The shield just regenerates. Hovering over the shield health it says that an EMP will bring it down but it simply does not. I was using EMP grenades, turrets and rifles.
And the boss is kind of cheap too since it will fire space lasers which penetrate your shields and take out all of your defenses. So building any turret is useless. It also has a shockwave attack which also penetrates shields and will destroy any building. So again, base defenses are useless.

You can survive an extremely long time sitting under a ton of shields, with shield backpacks and behind really strong walls but you'll never make a dent. Bring in a single melee pawn and it's GG.

I think make the 3M hitpoint shield not regenerate. I think that's enough of a challenge. That alone is nearly impossible anyway.

With mods, you can distract it with about 5-7 VOID mutants and send one really strong melee colonist to spam attack for a few minutes. I also used invinsibility on the meleeist first (you can even bring another psycaster to cast it on the meleeist) so it was really easy.

Without any mods besides Kraltech, I'd probably use a group of tanky robots from the mod as distractions instead.

For me, dealing with the army around the boss was more difficult, had to cheese them by forming a caravan and picking them off one by one