I've defeated like 15 Mothrons and not one has dropped a Broken Hero Blade. I've had about a dozen Death Scythes drop though and that's how I was finally able to defeat Mothron for the first time. Since then its been a piece of cake. I sit in my teleportation room underneath my lava trap until he arrives, then I teleport out and fly back and forth spamming on the Death Scythe and dropping down to platforms or flying up to my "roof" (to prevent accidentally spawning wyverns as I fly madly about) as necessary to avoid his lunge attacks. I'm able to avoid the majority of its attacks and take minimal damage. Once its dead I just fly back to my teleporter and back into my room until the next one appears. With the Water Candle, I get on average 3-4 Mothrons spawning per Eclipse.
Otherwise, Expert mode has been challenging, but besides for Skeletron I haven't felt overwhelmed, and I've never felt I could not do it. I have only had 3 attempts at Plantera and its made me realize I need to drastically alter my strategy and arena for this newer faster version. I've got 72 armor, pushing 80 with Ironskin, but my mobility needs to be improved.