Greetings fellow monster-trainers,
in order to defeat all monsters in
a single turn, you should kill all 3 enemies in the first turn.
I managed to get this achievement when my monsters got around
level 12.
I retourned to the
Mountain Path (first area, green colour on the map): enemies are about level 2/3 and have low HP. Pick your team according to the
weaknesses of the enemies and make sure that you have one attack in each monster according to the weaknesses of your enemies (for instance fire/wind/water attacks).
You should be able to kill an enemy with each of your own monsters.
Read more about damage types and weaknesses:
https://monster-sanctuary.fandom.com/wiki/Explore_AbilitiesRead more about each monster and its weakness:
https://monster-sanctuary.fandom.com/wiki/Monster_TypesOf course you can first use some buffs and/or items in the first round(s) and then kill your 3 enemies on turn 3/4/...Enjoy!