Killer Instinct 2 Classic

This Achievement basically translates to 'Retry 400 times'. You can do this with either one Controller or two Controllers, but the two Controller method is much faster.

One Controller

If you have one controller, you can start go into the Options on the Main Menu and raise the difficulty to the highest level and start an 'Arcade' (Local). Let the AI beat you, then Press A during the countdown to use a 'Credit', and repeat.

Non-Turbo Turbo Boost - Credit: FelizP: You can rubberband your controller's A button in order to automatically restart arcade every time you lose. If you have a wired controller or a Micro-USB to plug a wireless into your console, you can essentially Idle Boost this by starting Arcade and keeping the A button held. According to comments, even if your controller is plugged in, it will still disconnect from just having the A button held, therefore you'll need to be active and moving the Analog Stick ever few minutes to prevent disconnection.

Two Controllers

A much faster method for anyone who has Two Controllers. Press A on a Second Controller during an Arcade run with Player One; pick a character with your first Controller then hold and press which should display 'Spinal - Ship' in the centre of the screen. Pick Sabrewulf with your Second Controller then quickly hold and press once again. If you did this correctly, the centre of the screen should say 'Stage: Sky'.

On this special hidden stage, you can knock the Opponent out of the ring and win very quick; there is a slight disadvantage to being the Second Controller on this stage, since the default position is not in the centre and as such it's harder to Ring Out in the first few seconds. By picking Sabrewulf, we can perform a very simple combo on Player One that will knock them out within 2 seconds. Jump at the Opponent and Press then, as soon as you land, press + and then immediately input , + . The second RT press will be a two hit Auto, so you have more than enough time to input the final move; doing this combo will allow you to knock out the Opponent in 2-3 seconds.

Once the falling animations begins playing, press on the Second Controller until the game requires Player One to 'Continue' (You might be asked to input a name, but this should only work once), so press on the first Controller, then A once again on the Second Controller; this will send you back to the Character Select; from here follow the previous steps, picking the Sky Stage, and repeat.

This might seem complicated at first, but once you get used to it you can use tokens very quickly. It will take you 3-4 hours to grind all 400 if you use the Two Controller method.