If you want to get some of the coolest swords in the game, you'll need to get crafting items called Broken Hero Swords that drop from Mothron. Besides those, the Solar Eclipse has awesome weapons like the Reaper Sickle.
Mothron is insanely hard on Master Mode though, so I'll give you the best way to beat him along with farming for the many Solar Eclipse drops.
Arena
The first thing you need to do is to build a base with lava pits on either side. These will protect you from most ground enemies, and allows you to kill a ton of them without having to use weapons.
The only downside is that the lava will burn up the coins, but you can collect it quicker using a Treasure Magnet.
Recommended Gear
With the base set up, you need to craft a decent set of armor along with a good ranged weapon. I used a Hallowed Repeater, but an Uzi, Minishark, or another repeater will work well too. Then, use some good ranged boosting accessories like the Magic Quiver and Ranger Emblem.
As with several of my other guides, you’ll need the Black Spot mount once again, and that’s what we’ll be using to beat Mothron.
Summoning the Eclipse
To get the Solar Tablet Fragments to summon the event, you need to farm them in the Lihzarhd Temple, and you can do this literally as soon as you enter Hardmode.
I was very surprised to see that the Temple Door glitch wasn’t patched in Journey’s End, so I think they may have left it in the game on purpose. Using only 3 wooden platforms and a hammer, you can actually walk straight through the door.
Place the wooden platforms on top of each other touching the door, and hammer them into an L shape. Then, hold down and walk forward and you’ll walk through it. Once you’re in, try to destroy as many traps as you can. Loot the chests and kill as many enemies as you can, and you can craft a Solar Tablet when you have 8 fragments.
Invasion and boss strategy
To get the most possible time out of the Solar Tablet, use it as soon as daytime starts at 4:30AM. It can only be spawned in the day and lasts until nightfall, so this gives you the longest eclipse possible.
When the eclipse starts, the lava trap will take out most of the enemies for you. You may have to watch out for Reapers though, but you can trap them at the bottom of your base if the ceilings are high enough.
All you need to do is wait until Mothron spawns, and then you immediately leave your base and start flying with the Black Spot mount. Mothron will follow you, so start shooting it with your ranged weapon of choice. You’ll reach the end of the world pretty quickly, so fly up and go around him when you see the ocean biome.
It won’t take very long, but Mothron will die and drop some coins. If you’re lucky you’ll get the broken hero sword, but most likely you’ll have to beat him several times to get one. Mothron spawns several times every solar eclipse, so just wait for the next one to spawn.