What I have found is that it is essential that the audio be recorded directly into the iPhone. While external recorders can be later synced up in Final Cut, this process is super cumbersome and means that you can’t easily check your footage in the field. It is also essential to have some kind of wind/weather screen on your microphone, otherwise you will constantly be ending up with unusable shots where all you hear is wind noise.
[On it. Kinda. Sorta.]