first, if you want to be able to do it easy, be anyone. If you want an acheivment FAST or just want to show off to all of your friends, then be Yun-Sung. Use his attack Avenging Claws (>B,B and for you newbies >Y,Y). Do this when theyre blocking. next to there health is the soul gauge (colorful orb). You want to drop this all the way. Here's a list for the order that it's in: Blue (furthest)
Flashing Red (very close)
Soul crush (CRITICAL FINISH NEEDS TO BE USED NOW!)
ok. now to do this, keep attacking them when theyre guarding. then theyre soul gauge drops slowly. You can speed this up with certain custum skills but its not nececarry. When the soul gauge is flashing red and red is flashing arounf your health, use single attacks to make sure you dont overdue it, it takes perfect timing. now when the soul gauge is grey, and their gaurd breaks away (try not to think about what to do at that time, it lasts about a second.) Click at the same time, G, A, B, K (A, B, Y, X) or for the lazy people, RB (which works best). now witness an awesome short fight seen and enjoy your new acheivment. Seigfrieds Critical Finish is the coolest though.