Here is the full list of all 21 Xander the Monster Morpher: Universe Breaker achievements.
Catch your first monster
Catch 25 different monsters
Catch 70 different monsters
Caught every monster
Fuse your first monster
Defeat Molord
Defeat Archair
Defeat Volking
Defeat Abyssire
Defeat Tennet's Oculus
Defeat Oculus
Defeat Ardul
Get the true ending
Finish a Nuzlocke challenge
Finish a Randomizer challenge
Finish a Randomlocke challenge
Catch a shiny monster
Catch all shiny monsters
Help Carina clear her farm
Catch a rare fish monster
Clear the top floor of Deth's Manor