Here is the full list of all 21 Undertale achievements worth 1,000 gamerscore. It takes around 4-5 hours to unlock all of the achievements on Xbox One.
Entered the Ruins.
Donate 10 coins to the Dog Shrine, or...
Donate 25 coins to the Dog Shrine, or...
Donate 50 coins to the Dog Shrine, or...
Donate 100 coins to the Dog Shrine, or...
Donate 200 coins to the Dog Shrine, or...
Donate 350 coins to the Dog Shrine, or...
Donate 500 coins to the Dog Shrine, or...
Line up three ghosts in the Dog Shrine, or...
Line up D O G in the Dog Shrine, or...
Line up three bones in the Dog Shrine, or...
Line up three cherries in the Dog Shrine, or...
Reach the mouse hole in the Ruins.
Arrive at Snowdin.
Get to the Spaghetti save point in Snowdin.
Arrive at Waterfall.
Reach the place where it rains.
Enter Hotland.
Reach Hotland 2F.
Reach Hotland 3F.
Reach the second save point in the CORE.