"A Fine Blade" is obtained by getting the Blade which you get after finishing the Sand Zone. "A Gift From Santa" is obtained by getting the Fireball, which is given by Santa of Grasstown as thanks for finding his key.
#TINYKEEP SECRET SHOP SERIES#
"Return the Favor" is somewhat tricky - you have to unlock the secret dungeon by completing a series of tasks from Labyrinth onwards, then just get to a certain point in the dungeon (it's early on, in the first room, when you attach Curly). You'll pick up the item when you wake up. All you really have to do is talk to her (check the fire at her house for the first time), and then sleep in the bed. "Ladies Man" can be obtained by sleeping with Chaco in Bushlands.
You'll know you got it if all the sprites (and levels) are holiday themed - there's no popup for this achievement for some reason. "Merry Holiday Happy Euphemism" is obtainable by playing the game during December 24 or 25th OR by setting your computer date to December 24th or 25th. "Happy Birthday Pixel" can be obtained by playing the game on April 29 or just set your computer date to April 29. The Spur and Booster 2.0 is useful against Ballos' first 2 (and a half) forms. The Whimsical Star helps a lot with the archers in B3 of the Sanctuary/Sacred Ground/Hell. The 290 Counter (formerly the Nikumaru counter) is for the speedrun/time challenges (to unlock new tracks and characters on the title screen). Only the Booster 2.0 and the Iron Bond (by rescuing Curly) are required for the Best Ending (which you get after beating Ballos in the Sanctuary/Sacred Ground/Hell). Some achievements might not pop up, so check for your profile to see if you obtained it. Let me know if any corrections are needed. Here is a compiled list of the hidden and some of the more cryptic achievements, from both personal experience, this list, and research (Googling, asking other ppl, etc).