Pick any enemy soldier from Free Play, faster than "Motorcycle Escape" or "Opening the Ark", go to "Ancient City" instead. Ancient City is a bonus level that is unlocked after you have collected the artifacts for Temple of Doom.
I was averaging 4 to 5 gates every 2 minutes VS 3 gates every 2 minutes on the other possible levels.
I can confirm this can be done as soon as you use the enemy gate on the right of the Ancient City map, exit back to the museum, jump back in, repeat ad nauseum.
It will save your count don't worry.
I hope this helps someone, I thought it was extremely fast and easy.