Load up 1-5 “Catch of the Day” with invincibility mode turned on (go to options, -> assist options to turn this on and off).
Progress through the level until you get to the section where you use a cart to break down a gate.
Just run around this section using only the fish to get kills, and once the final enemy dies just pause, and restart area. can average around 10+ kills per run thru, should take no more than 5 minutes of grinding.