I found the easiest way to do this was on the manbearpig boss level. You get one Core for beating him and one for the gold.
When the level starts immediately jump over manbearpig to the right causing him to charge at you. Quickly be prepared to jump over him again to the left and grab the box. Run with the box and throw it at him. You can jump and throw it at him you don't have to run right upto him.
When he gets back up avoid the falling obstacles and stay aound the middle of the bottom ledge, jump him again when he charges, get the box and throw it at him. Do this one more time.
Once you hit him for a third time, climb the ladder and wait in the center of the second floor, after the cut scene walk into the time vortex.
The gold time is 45 seconds.