I've seen so many bosses being created throughout various maps.

For example, in the map Das Herranhaus, how does the creator even start to begin with making Hitler as a BOSS.

I guess he modeled him in blender or maya right? How is this process done, to script and code a custom boss, an enemy. I know I am a new map maker but I have been doing it for about a year or so now. Rather than focusing on just making a map, I know that already. I know the basics, I wanna learn more advanced things. Can someone explain how I can learn to make such an enemy?

thanks