Here is the full list of all 31 Asterix & Obelix XXL 3 - there are The Crystal Menhir achievements.
Recover the Crystal Menhir
Finish the game in “Easy” mode or a higher difficulty setting
Finish the game in “Normal” mode or a higher difficulty setting
Finish the game in “Difficult” mode
Finish Chapter 1 - Gaul
Finish Chapter 2 - Thule
Finish Chapter 3 - Tyre
Finish Chapter 4 - Crete
Finish Chapter 5 - Rome
Finish Chapter 6 - Save the village
Defeat 500 Romans
Defeat 1,500 Romans
Stun 250 Romans
Freeze 100 Romans
Warm up 100 Romans
Defeat 500 Romans under the effect of a magic potion
Throw a Roman and hit 10 enemies
Send 6 Romans flying with a single uppercut
Throw 100 Romans
Stun 250 enemies with Romans falling back to the ground
Throw one Roman back and forth 5 times with another player
Do a 50-hit combo
Do a 100-hit combo
Do a 200-hit combo
Defeat 10 Romans with a barrel of powder
Make a Roman hit a target
Possess 5,000 helmets
Collect 10,000 helmets
Buy a character upgrade
Buy all the character upgrades
Complete all the camps with a gold medal