... here's what I could think of.

Kindly King: Buy the pelts from the man in the Forest Perilous.

Boar Gore: Kill the Boars.

Joust Jock: Defeat the Black Knight without getting hit once.

Skating on Thin Ice: Pass over the ice on Lake Ot Moor without breaking any ice, or using any items to help you.

Flower Power: Complete the Trial of Flowers without making a single mistake.

Riddle Master: Complete the Trial of the Goddesses without making a single mistake.

Saracen Shish-Kabob: Defeat the Saracen without getting hit once.

Soul Man: Earn maximum Soul Points.

Fightin' Man: Earn maximum Skill Points.

Wise Man: Earn maximum Wisdom Points.

Iron Man: Earn maximum points in all 3 categories.

Sir Robin the Brave: Sacrifice Gawain but no other knights.

Hoes Before Bros: Sacrifice Launcelot but no other knights.

Self-Preservation: Sacrifice Galahad but no other knights.

He Chose... Poorly: See both fates of the Thief.

Arthurian Legend: Complete the game.

Oh My God, You Killed Camelot! You Bastard!!: Get the bad ending.

It's a Sad Thing Your Adventures Have Ended Here!: Experience all deaths.

Anybody else have any ideas?

Skating on Thin Ice and Saracen Shish Kabob would be very hard achievements to get. Also, if you sacrifice any of the knights, you die at the end of the game. So that'd be a strange thing to get an achievement for. Lastly, I don't think this game has a bad ending besides death.

Some that I would add:

Cute little puppy: Make Merlin turn you into a puppy.

Bye Bye, Guide: Kill the fake tour guide

Monty Python: Find the Monty Python Easter egg

I'd call the achievement for finding the Monty Python easter egg "Tis a Silly Place"