Stokers are the very large zombies carrying shovels that look like hardened magma. Not to be confused with the fire elemental zombies that detonate when you kill them.
After damaging the Stoker a certain amount, a weak spot will be revealed through a glowing yellowish-orange spot on either side of his chest or the top of his head. Shooting the glowing spot will cause him to drop to his knees and blow up from the weak spot.
Later rounds are recommended for this, as they spawn very frequently the higher you are. I did it on Round 32. There were about 6 or 7 across the entire round.
EDIT: WayzoftheRaven added that using the Kraken did not unlock the achievement, so it would be best to avoid weapons with explosive or area-of-effect damage.