Very easy to do but be prepared for a frustrating few minutes!
You need to find the Hidden Battle with Klassic Reptile.
Firstly make sure your round counter is set to 2 in the options menu. Now start an Arcade Ladder, i choose Sheeva for 2 reasons.
1. Her (Down, Up) stomp move & 2. Her easy to do Stage Fatality (Down, Down, Down, Down, X, close distance)
UPDATE - l've had a question on if changing round time counter affects this achievement, answer is I don't know. I would suggest ensuring all options are set to their defaults to be safe and not waste anyones time. If anyone does have an answer to this question please post within the comments thread!
You can choose whoever you like but i recommend someone you are familiar with and can perform their Stage Fatality.
Then select Beginner on the Difficulty Select
Now allow the Arcade Ladder to start and drop down to the first fight. Next to your opponent will be a pitcture of the first stage. This needs to be The Pitt at Night (Look for the large moon). If not let the match start then pause the game and select return to Main Menu. Then rinse the solution from the top. (THIS IS THE FRUSTRATING PART AS GETTING THE RIGHT STAGE CAN TAKE SOME TIME AS IT'S COMPLETELY RANDOM)
Once you get the right stage and the match starts the are 3 factors you need to achieve to fight Klassic Reptile.
1. Get a Double Flawless Victory
2. Perform the Stage Fatality
3. See a shadow of either Santa Claus' Sled or A Witch on her Broomstick fly across the Moon (another throwback to the orignal MK)
Use Sheeva's Stomp Move (Down, Up) to get 2 Flawless Victories, but keep an eye out at the moon at the rear for either shadow (Usually quite early on). If you get to FINISH HIM/HER and have not seen the shadow DO NOT PERFORM THE FATALITY!!!
Pause and select return to Player Select. Re-Select Sheeva (Your Character) and the match will repeat at the same stage and opponent. Repeat until you see a shadow and the Perform the Stage Fatality. (Also do this if you get hit during the fight & can not then get a flawless win)
After the Victory voice over you will get a loading screen and fight Klassic Reptile in the Pitt Bottom like in the Original MK, Achievement unlocked.
Beat Him however and recieve 5000 Koins per Round
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