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