In China, I'm a practicing lawyer and arbitrator. In Canada, I'm a licensed immigration consultant, an international arbitrator, a certified Mandarin-English interpreter and a Chinese language teacher.
The most difficult part for learning Chinese is that Chinese phonetic system and written language don't reflect each other as English do. For Non-native Chinese learners to master Chinese, I recommend learning and memorizing Chinese Pinying and strokes as the start. The challenges facing the intermediate learners usually are vocabulary and grammar; therefore, to help them apply what they've learned in real-life settings, from my perspective, is the best teaching approach.
As for the advanced learners, they know where they need to improve and likely, they know what they want to achieve; thus, I can tell them how-to, make a plan for them and help them along the way.