After you get the good ending the dragon will unlock. In the present ride it to the basketball court and spit fire on the black cows. They will transform into burgers and after 4 you will get the achievement.
If you are interested in a very fast guide :
Timestamps:
00:00 Intro + Self-sacrifice Achievement
00:59 Story Present + Town Musician of Bremen Achievement
04:08 Story Future Ending 1 + 13th Deer Achievement
07:23 Story Future Ending 2 + The Trudeer Show Achievement
08:49 Counting Sheep, Deer-hunting Achievements
09:18 Dead or Deer Achievement
09:33 God Disguised as a Cow, Kept you Waiting? Achievements
10:19 100% BEEF Achievement
10:47 Like a Truck, Dunkleosteus Achievements
11:20 The Devine Move Achievement
12:30 Genius Achievement
13:19 Rhinosaber Achievement
13:49 Big Time monkey. I am Deeeer Achievements