Here is the full list of all 16 Hexaduel achievements.
Win a duel after surviving a 5-digit hex
Win a duel by depleting the enemy’s HP all at once
Win a duel after casting 3 perfect wards in a row
Win a duel after surviving a sudden-death hex
Win a duel with 1 HP left
Win a duel while having exactly 12345 MP
Win a spar on AI level 3+ without using any magic
Win an advanced spar on AI level 4+
Win a spar on AI level 4+ with a cast time below 7
Win a spar on AI level 5
Win a gauntlet duel while having over 65536 MP
Pass the gauntlet's fifth stage
Doing 0 damage, score 24000+ in the trial of mercy
Reach a total highscore of 160000 in bonus mode
Win a duel on a Saturday after 9 PM
Withstand the master