Make sure your squad consists of a ranger, and someone with a flashbang grenade. The ranger can carry the flashbang, but it will take an extra turn to get this done, since throwing will use his turn. Luckily the flashbang disorient effect lasts for multiple turns.
Do not try this without disorienting the berserker first! He will counterattack your swing, and you will do no damage.
The hardest part of this achievement is actually finding a Berserker! I found them most often on the "Save civilian" missions. Once you find one, lower his health, toss your flashbang, then melee with the ranger.