This achievement is the worst in my opinion. It depends on random luck. You have to get through 2 specific events in the concealed caverns in the right order. The first one is the one that proposes the "petty theft" spell among two other options. Take petty theft. Then you need to pray to get the right other event in an other concealed cavern. This second event is the one that proposes 2 chests (options) that harm your spire, gaining gold in the process, respectively 25 gold against 5HP or 150 gold against 25 HP I believe. If you have the petty theft spell there will be a third option to get 175 gold and no HP penalty but the spell will be purged, this is the option that will trigger the achievement.
Now to put all the chances on your side you should :
- Pass through all the concealed caverns available, 3 or 4 depending on the runs. No need to go for the last one if you did not get petty theft prior to this.
- Go on difficulty 1/25, this will allow the easiest ans fastest runs.
- Blitz all the fights to gain time, you don't want to spend 45 minutes per run, you will go crazy. My choice to speenrun the fights was to start with the umbra, the other clan does not really matter but the hellhorned was the most convenient to target fairies with torch spells. You should begin with the umbra spell void biding and the umbra hero that has trample. Until you get this specific combination, start over. There is a button to start over in the menu with the same conditions that will save you some time. For the runs themselves, you need no units. Improve void biding with holdover and doublestack asap and duplicate the spell as much as possible. Purge the other cards. Most of the time this allowed me to start over at 11-12 minutes before the second main boss. If you have faster ways, by all means go for it.
Good luck guys that's all it is about.