Go to the Imperial Seal level [I posted how to get there in one of the hints below this one it is the first secret level you go to] And defeat every officer and troop [Or was it just officer] Anyway you might want to do both for two reasons.
#1: I'm not sure witch one it was.
#2: You are going to be waiting 30 minutes because the Imperial Seal wont appear untel there is only 1 minute left untel you lose and it will apear in the top left corner of the map if i remember right. If it is not there wherever it is when it appears the location of it will be pointed out on the map the second it comes in.