TEKKEN 8

Easiest and fastest way to perform a floor blast:

Set two rounds battle PLAYER vs CPU, on easy, and select Jin. Then select "Into the Stratosphere" stage. You need go to the first floor with flames and lava. It hapenns in second round, so win the first one. Then just switch to special style (tap by default) and catch your opponent into special throw, press and hold + (BACK+B). Once the floor explode achievement is yours.

//Edit:
Just checked. You can use Law basic throw (+) instead of Jin special throw.

“Floor Blast” can only be performed on the
"Into the Stratosphere" stage.

The easiest way to get this is to play Story: The Dark Awakens > Chapter 14. In Chapter 14 you play on this stage as Angel Jin. At the start of the fight toggle on “Special Style” ( LB ) > press right direction key + X (standing left). This will perform a grappling move. Angel Jin will fly up and smash the opponent on the ground, unlocking the trophy in the process.

This is obtainable in Arcade Quest / Super Ghost Battle / Character Episodes / Story. It’s not obtainable in Offline VS / Practice / Replay.

For those who prefer video guide: