Just kill 400 nazis in Little Berlin.
This is the fastest way to mastery up Laserhammer! Choose easy level of difficulty then go to Little Berlin. You have to enable invisible and run through enemies. There is a stationary Laserhammer near the crossing point. Just go to it and kill so many nazis as you can. Do not pay attention to Panzerhund, you can kill him at the end. After killing all enemies just go back to catacombs and do the same. You can kill about 10 nazis at once. More details in the video. Good luck!