i think 100-150k range will do for intructors. As for the easiest one, i dunno. But the most difficult one is mihile, then the blaze wizard( dont remember the name). So its either Echart,hawkeye(?) Or the Striker.
Hawkeye is by far the easiest, but only if you're ranged.
Eckhart is easy if you can deal with shadow web.
Irena is annoying because she can outdistance many ranged classes.
Mihile has DR.
Oz has seal.