Some suggestions I thought of because I'm on a plane.
Secret Weapon - During a boss battle, switch to a weapon that deals more damage than any other weapon you've used so far in the boss battle. (Optional: Kill the boss with the weapon)
<I haven't thought of a name for this one yet> - Use weapons to kill a boss while using accessories and armor meant for a different "class".
Ammo Cheapskate - Shoot with a gun for 5 straight seconds without using more than one bullet (Ammo boxes, potions, Megashark, etc..)
Music Master - Own every single music box (Including title screen (and any other secret music boxes) and new music from the 1.3 update)
<Not sure about this name either> - Sell one item for 1 or more platinum coins including non platinum coins (1 platinum and 1 copper, etc)
Irony - Kill the king slime using only the slime staff, the Queen Bee with only the bee gun/wasp gun, duke fishron with only the swordfish (or any other fish-related weapon), etc..
Lawnmower - Cut 1000-X grass type tiles (Vines, grass)
She sells sea shells by the sea shore - collect X coral/seashell blocks from the ocean.
Meat Grinder 1* - Kill 10 enemies without moving
Thriller - Be a werewolf.
2spooky4me - Get spooked by the dungeon guardian
hop u laeik diiz sugjeschuyions!?
*Meat grinder 2 would just be the number of enemies needed to be killed in the first one multiplied by 10 (same with meat grinder 3,4, and 5)