It's really hard to get an A on a philosophy essay because there is always something for the TA to rebuttal. My best advice when writing philosophy papers is to think of every possible opposing view, mention it, explain it, then state why your view is better/makes more sense.
Make sure your writing is also clear. There's no need to use big words all the time, sometimes they just make your essay sound more confusing than necessary. Stick to the page limit/word count, and make sure you have a very clear, and if possible, unique and original thesis. Don't forget to state a brief summary of your argument in the intro, or you'll lose marks.