What is your gear situation? The biggest issue with Mistsong Hard (asides from people who toaster when it comes to Aria) is dps. There is a 10 minutes dps timer for each boss that you must meet. Taris (the hard boss) is a dps race. So the more dps the better.

Team composition is everything! Play with team composition and find what works best. Remember that unless you're aiming to farm hard mode, pick a team composition with good safety margins. My farm group is composed of me (melee for bosses, mage offspec for Taris), an argent (archer offspec), two mages, and one cleric/healer. This composition I personally wouldn't recommend since unless your healer is good and doesn't mind chugging mana pots to make up for your team's mistakes. Also having an off-spec that meets the rough dps requirements is rather expensive. So find your ideal spot. Just remember if you want to run Mistsong hard, DPS DPS DPS.

Mechanics do change for each difficulty. Easy is a good introduction to the basic concepts of each boss. Each difficulty increases the frequency of each mechanic, may be more erratic in mechanic pattern, and of course, have more health/be tankier/do more damage.

I would have to write a booklet worth to cover the bosses each, but TLDR: Aria (medium) - She will do the tile mechanic (where the ground flashes and you have to find the right tile) more often, in addition to randomly summon adds (not referring to "Love is an Open Door." Medium should feel like a harder version of easy.