1. get 717hp
2. use pergea 08rs tires
3. use red suspension and lower the ride height to the minimum (don't use skirts, they raise height, hindering acceleration slightly)
4. search "How to get 1:20 in Gunsai TA" on youtube and watch the video
5. practice until you get under 1:25
make sure you use tilt or slider, arrows are a no-go.
after some time you can probably wipe off a couple more seconds easily, then try to get below 1:21 in the future.