How should I write a eulogy from Macbeth's point of view talking about his wife if I cannot discriminate against or diminish any of the characters?...

Readers sometimes have a tendency to belittle Lady Macbeth, insisting she is totally evil or crazy, and so I imagine it is statements or sentiments such as these that you are meant to avoid. Macbeth would likely not make such claims about his wife. I would recommend that you try to identify the qualities Macbeth would most admire in his wife. At the beginning of the play, at least, he seems to greatly respect her, calling her his "dearest partner of greatness" in his letter and hastening home so that he can confer with her about what he should do. It's true that she held him to very high standards of conduct, especially in regard to keeping his promises, and she has never been afraid of calling him out on his faults, but you could certainly make the case that she does so in order to help him to become his best self (if you believe Macbeth would feel that way). It seems likely that he would credit her, at least in part, with his rise to power since it was she who coerced him to murder Duncan in the first place. Consider what Macbeth seems to value, and then try to assess how much he would feel his wife has helped him to acquire what he values.