To kill Shiela (red dressed @#$) easily, you just need to buy the pit. Time it so that you've cleared most of the current enemies by the time she comes out. Approach her, then smack her to the pit with the chainsaw. You can even knock her to the pit using the boomstick. One shot death.. and it will net you an achievement because Ash also did this to her in the movie... nice touch by the devs.