Time stops need to set up by hand, there really isn't a way to do it with one simple task/function. Really, time stops are composed of three parts:
1: Stopping all the bullets
2: Stopping the player from moving
3: The boss does something interesting
4: Starting the bullets moving again
Stopping all the bullets is fairly easy: Get an array with all the bullets in it and then set their speeds/accelerations/angular velocities to zero. You also usually want to store the old values in the bullet using Obj_SetValue, so you can start them again afterwards.
Stopping the player is more difficult. You can stop them from moving by continously setting their location to where they were when the timestop started while the timestop lasts, but this doesn't look very pretty[The player can still jerk around]. The other way is by implementing an event in the player script that sets their focussed and unfocussed movement values to zero[Remember to restores them afterward!], which works best with a custom player script.
Step three is where you actually do interesting stuff. This depends on what you want to accomplish with your timestop. General advice: Make sure that any bullets fired in this stage also start out timestopped.
Starting the bullets again is just step one, but in reverse. Here's where storing their previous speed values becomes important.
None of this is very complex to implement, although you do have to account for some potentially hairy edge cases[What if a bullet is destroyed during the timestop, what if the timestop ends prematurely, what if the player dies during the timestop, etc]. Timestopping can be very messy if something does go wrong, so playtest it thoroughly.