1.She's above telling a lie - She never tells a lie.
"She never tells a lie" is, presumably, a statement of fact, whereas "She is above telling a lie" emphasizes that this is a particular moral strength of hers.
2.He is a good mixer - He is very good at socializing with others. OK
3.You know that I'm as good as my word - I never say anything not trustworthy, I only say the truth.
Most commonly when a promise is made to do something in the future: "I said that I'll pay you back on Monday and you know I'm as good as my word".
4.He is on top of things- He always does things perfectly.
More often, well organised, highly capable, well informed. If he also does everything perfectly then hire him!
5.His bark is worse than his bite. - His nature is not as bad as it seems.
not a teacher
Student or Learner