就你的这个例子,我想你可以这么说: I see your points, however, .... (mention some examples that you think she deserves better. Make sure give facts not just perceptions.) ...
And then give it a positive spin, say something like: "What we would like to see is she improves and do better job, which is better for both of us. Some appraises and encouragement go long way. I am sure she will appreciate it and try even harder."
如果你不能找出 facts 的话, 也许你的 perception 不是太准? 只是觉得"6尺5"太凶了.