Alternatively you can just go to "Extra -- Unlocked Level Codes" and load Dimension 15 (you need to have all 3 large Squares of it collected, which shouldn't be a problem). Now click "Save Level" (via the File menu or the button next to "Play Level") and click "Copy to Clipboard". With this, you've copied the level's code. Now open any text program (Word / Writer, Notepad etc.) and insert the code. It contains all the info of the level (dimensions, tiles, items, course properties). Every item has an ID that makes it recognizable, there's a complete list
here. In this list you'll also find the boss at the bottom, although it's hidden (not included into any Items menu).
Also, each item is represented via its own segment in the code. These segments are separated by "|" and contain all of the item's settings (coordinates, tag, size, triggers, and other specific stuff etc.). Since the boss item is hidden, you must
insert it into the code. That's why you copy-paste it into e.g. Notepad. Now put the segment above next to another segment (add a "|" so there's two "|" next to each other and put the rest in).
Now re-load the code into the designer. You'll find the boss at the coordinates you've inserted (in this example, x = 3632 and y = 2224). You can also change its amount of health to whatever you like.