Before you attempt this, I would wait until you either complete Leon's campaign or play until you get the magnum with plenty of ammo. It is possible to defeat the Lepotista (thing that comes out of the front stand with hives) on your first normal/ amateur play through but having the magnum makes it so much easier and quicker.
During the cathedral scene after the puzzles,(2-2) the hive thing will come out. When it does its blue cloud attack anything around it becomes a zombie thus killing them. You need to keep it away from the other survivors as best as you can. Using the magnum and rifle makes quick work, making it stutter. On amateur it is about 8-10 magnum shots to kill it.
The achievement should unlock at the end when there are at least 2 women that had survived. BTW, Helena does not count as one of those women.