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