Here is the full list of all 23 Undefeated achievements.
100,000 gold found
Giftly Shrine Visited
All 25 secret rooms
Genesore Slain
Treesore Slain
Lurker Slain
Spoiled Rocks Slain
Genu-Geko Slain
Mutant Raflesia Slain
13 secret rooms
Cracilia Slain
10,000 gold found
5 sidequests
Beriberu Found
Sporarum Slain
1,000 gold found
3 secret rooms
Kingly Lizard Slain
1 hour played
Magasz Found
20 hours played
Finished in less than 6 hours
All 16 sidequests