Hey @EmirhanOnuk! Welcome to the forums!
With the information you’ve given, I can suggest extending the 200 unit distance- when determining distance, it draws from the origin and seeks the other actor. So depending on how large the boss is, you have to add more distance to accomodate.
For any further help we need more information. You say he only attacks while he’s next to you- are you trying to make a ranged attack? Is the problem that it isn’t moving up to be within range?
If you need more assistance, let us know! 