Tips for Killing Chaos Spawn
You have to act against your instinct to focus down the Chaos spawn and instead kill all the cultists first, so that it cannot heal itself.
This may require you to employ a counter-intuitive deployment strategy- e.g. placing your range units up front, so that they can start off in line of sight of the cultists (annoyingly, a number of them are off to the sides and can’t be targeted if you place your shooters in cover or far away from the Spawn).
Even with the right strategy, there is a chance for the fight to turn against you if you’re unlucky. For example, to get them into line of sight with the cultists, I had to place my PC (Sanctis psyker/soldier), Argenta, and Indira up front- just behind Abelard, who was directly in front of the Spawn.
Thankfully, all the cultists only require one good hit to go down. And it seems like you start with a Momentum boost- i.e. in round one, I was already able to activate my soldier’s Heroic Act, which allowed him to shoot 5 times and take down many of the cultists.
However, the Spawn proceeded to ignore Abelard and focus on my soldier, basically knocking him out of the fight from round 2 onward. Thankfully, with the cultists gone, it doesn’t take long for your remaining 3 characters to deplete the 140 HP of the Spawn.
In subsequent reloads, the Spawn sometimes targeted Idira and sometimes Argenta, Rarely did it target Abelard, even though he was the closest. The fight was frustrating in that regard. But when the Spawn did go for Abelard, it could be locked down effectively, as that character (with Endure and Idira’s protective power cast upon him) can tank the Spawn’s multiple attacks.
I was playing on Daring difficulty and did not find the fight hard, just messy, as you needed luck to avoid injuries on your squishier characters. It wouldn’t be so bad if you could keep them in cover but the nature of the fight forces you to put them in harm’s way. It felt like a damage race.