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I will use Epic Boss Fight as an example to tell you workaround for this solution. This is a very cumbersome workaround, but it does work.
1) Go to "Steam\steamapps\workshop\content\570\305278898" (570 folder is Dota 2, 305278898 is Epic Boss Fight).
2) Create a folder in "Steam\steamapps\common\dota 2 betagame\dota_addons" called whatever you want. Pick something short like "ebf".
3) Download GCFScape to extract the files:
4) Extract the contents of 305278898.vpk into the new folder "ebf" that you have created.
5) Run Steam and start Dota2
6) Open Console (try with ` or \ key) (if your console doesn't open, then follow up coming console guide)
7) type "dota_launch_custom_game <map_name>" without quotes. Folder name is the folder you created. Valid map names for Epic Boss Fight are epic_boss_fight_normal, epic_boss_fight_hard, and epic_boss_fight_impossible.
Another way is to extract map and then load it up using the Workshop Tools. If you want to use Workshop Tools then
8) Run the Dota 2 Workshop Tools and select the folder you created. (If you don't have Workshop Tools, right click Dota 2 in your library list, go to properties, then DLC tab, then check Dota 2 Workshop Tools DLC. It's about 440 MB. You can run the tools by right clicking Dota 2 in your library and selecting the tools option).
Press backslash \ key to open console (if your console is not setup yet, then follow the instructions below)
Now you can open up console with backslash \ key