Any character can land a Lethal Hit, but it's easiest with Ivy.
Outside of Training mode, play as Ivy and attack your opponent with Square (A) twice. Repeatedly button mashing Square (A) will work. You want to intentionally miss the first hit. If the second hit lands without being blocked, it will be a Lethal Hit. A text prompt will appear if you attack was successful. After 10 lethal hits, the trophy will pop.
I played Arcade mode on Easy and got 10 hits within the first two fights. My video linked below: