Remember that in writing these papers, your aim is to create reasonable doubt. First, focus on what you think is the strongest argument from some author we've read in class and explain it highlighting what you take to be the author's crucial empirical and ethical assumptions. Second, explain how one might reasonably resist accepting the author's conclusion. In doing this, you might challenge their assumptions. You might also challenge the way in which those assumptions are being used.
When playing defense, you needn't satisfy the 'Matlock standard' (I hope you all have seen Matlock once). Anyway, Matlock, for those who don't know, was a defense attorney who would always defend his clients by discovering the criminal's real identity. This is one dramatic way of defending one's clients, but given the rules of the game, all you have to do is create some reasonable doubt about the guilt of your client. Similarly, in writing these papers, your task is only to create reasonable doubt and you can do this without satisfying the Matlock standard and proving that your view regarding abortion, euthanasia, stem cells, animals, etc... is the right one. In fact, I urge you to refrain from doing that as more likely than not, this will stand in the way of providing a careful discussion of the text. Instead, aim at satisfying the 'Psychoanalytic standard'. You are trying to imagine an audience that has been convinced by the reasoning of the article you are discussing and you want to undo the effects that article has on the reader. To do that, you must convince them that they were taken in by a line of reasoning they shouldn't have been. Telling them that the conclusion they drew was mistaken isn't really going to do much for them, they want to know how they were misled. You are supposed to tell them.
The Shabby Pedagogue: how to write an examination paper on moral philosophy. Here.
(An Argument Against Doxastic Voluntarism, again, is available here.)