The application of PCG to generate puzzles offers great value since their replayability is severely limited, requiring any game that employs them to produce many different puzzles. In this paper we propose a modified version of the progressive content generation approach to function as a mixed-initiative system, to create puzzles for the novel partially physics-based game \textit{Everyone Must Die}. Thus exploring the adaptability and usefulness of the progressive content generation approach for a unique type of puzzle game. Further the mixed-initiative system is explored in relation to how effectively it can generate puzzles with a specified difficulty, an issue many papers exploring puzzle generation neglect. This is explored by implementing and incorporating a PCG system by extending an existing puzzle editor featured in the game. The analysis is conducted with the help of a user study on the developers of the game by testing qualitative experiences with the system. The promising results are then discussed and concluded with suggestions for future work and improvements to the described system and its used approach.