When a Chosen appears, do not kill all his adds. He will "Wrath" summon more. Kill him first.
Do you have any enabled (like Alien Hunters or Shen's Last Gift)? xcom 2 long war guide
LW2 introduces 8 distinct classes (Sharpshooter, Shinobi, Ranger, Assault, Gunner, Grenadier, Specialist, Technical), each with three branching perk trees. When a Chosen appears, do not kill all his adds