Before you begin, you must have access to the Islands and I recommend you have a portal to Crystal Caverns as well as the Black Katana and double barrel shotgun.

From Caves 1, begin by grabbing the Island map piece from the left pedestal, Grab your starting weapon if you have that unlocked as well, begin exploring the caves, you're looking for the Black Katana (It has a green stripe) and a shotgun, preferably the double barrel shotgun. Once you find them take them to the shop and enchant them, I recommend playing as Vanguard or Spectre or their upgraded counterparts if you have them. Vanguard helps with free heals and weapon durability, spectre has the speed and free enchants.

If you've searched through cave 1 and you haven't found the green katana or the Shotgun, you can make do with a high tier melee like the flaming spear or regular katana, the only viable alternative to the shotgun is the harpoon gun.

Once you're all set, head through the blue portal shortcut to crystal caverns at the start of the area, once here you can search for the black katana or the shotgun if you don't have one yet, avoid taking damage and rush through the level to find the pedestal that matches the one we got the map piece from, this will take you to the Islands.

I ended up farming in Islands 2, but Islands 1 may work just as well, at least 1 shard lord should spawn, if not save and quit, avoid enemies when possible as it appears saving and quitting doesn't restore your health, healing and then saving & quitting doesn't seem to do anything either, I guess it simply takes your lowest health at any point and starts you with it, so really avoid getting hit to maximize farming duration. Take advantage of the increased jump height, you can jump to nearby islands as long as it's not the island the aberration (The final boss of Islands 2) is on.

Luckily, your ammo appears to refresh every time you save and quit, so make a beeline to the 1-2 shard lords that spawn, shotgun them ded, save & quit, rinse and repeat. All the shards you've gained will stay with you, when you're done farming, jump off the edge of the map, return to the facility and spend em. Note that if you find yourself falling off the edge prematurely you can save and quit before dying and start over.