For this achievement, you must beat arcade with every character in the game, including Seth. The computers, aside from Seth, are not very good at avoiding focus attacks, so spam focus attacks with each character, and follow up with a crouching heavy kick. Using this, you can easily get to Seth without too much trouble. Once you get to Seth though, you are going to have trouble, especially with characters you aren't familiar with. Guard constantly, and whatever you do, don't let him knock you down. You can use as many continues as you would like, as long as you finish arcade mode with that character.