First, draw your gun and face Boaz. You must get her to attack you head-on in order to get her into the mode where she rocks from side-to-side and shoots balls of green slime out of holes in her sides. These four holes (2 per side) are covered by "lids" and these lids are your targets.
The idea is not to shoot her in the face or to shoot directly into the exposed holes (both of which give you a satisfying splash of green blood), because this won't kill her.
You will be able to break the first two lids fairly easily because Kurtis will aim at them. The second two (the back ones) are harder because he keeps aiming at the front ones (or her legs, or the walls, or anything else in the room).
The secret is the button with the Orange Square on it (PS2). Once the first two lids are broken, tap that square button while firing to force Kurtis to re-target. He will then shoot at the rear "lids" and break them.
Once all four are broken, she dies (the first time).
You should have no trouble finishing the level after that.
Good Luck!!!