The hardest part about this one is actually getting the berserkers to spawn after their initial appearance, they seemed to show up for me more on retaliation missions. Just have some soldiers weaken it and then have a Ranger with a Sword or
finish it off. If you're having a difficult time, I haven't been able to verify this myself but if you light a berserker on fire or disorient them they shouldn't be able to use their melee (only) attack.