You can easily get this if you get Barricade early and face off against a non-scaling (doesn't gain strength enemy). Just don't attack and keep Defending after using Barricade.
I got Barricade off the bat using a Transform when selecting my starting prize. Then faced a couple enemies and narrowed it down to a Louse on like the third fight. Took about 5 minutes to build up.
Barricade + Entrench and small deck can get you to 999 in no time as well.
Not sure if we can link but here is a decent guide for this game: https://steamcommunity.com/sharedfiles/filedetails/?id=14603...