I prefer the eight point long handle L shaped tuning key. These seem to give me more leverage. I have a T shaped wrench with a square socket, that came with one of my 'harps, but prefer the long handled one.
I have trouble finding a good place to fasten the clip-on tuners to the 'harp, but the red Snark has a mic option and it doesn't have to be clipped on to use this.
Before the advent of electronic tuners, I used to tune to an "In Tune Guitar". The range of a standard chromatic 'harp starts at the F on the first fret of the 6th string and goes to the C at the 20th fret of the first string. I found it easier to tune a plucked string to another plucked string. I try to budget about 15 minutes to tune a 'harp, unless it's badly out of tune.