Soulcalibur IV

A soul crush is the red sparks that you get when you smash someone's soul gauge. The easiest way of getting this is to use Yun Seong on floor 11 ascending the Tower of Souls. Spam his "Towards and Y" attack and eventually you will clock up 100. It takes quite a while but you can help yourself along to the 100 Critical Finishes achievement at the same time by hitting LB when the red sparks fly.

There's a really simple way to boost this with a friend. Go ahead and set up an Online Special Versus Player Match. Whoever wishes to work on Soul Crushes should create the following Yun-Seong (credit goes out to Fatboyy for a nearly identical Yun-Seong, minus the Sorghum and some skills):

Weapon: Sorghum
Head: Horns of Calamity
Face: Usurper's Mask
Neck: Juok Chaplet
Shoulder: Paladin's Pauldrons
Undergarments: Executioner's Fauld
Upper Body: Noble Garment
Arm: Vagrant's Kote
Lower Body: Samurai's Hakama
Ankle: Shinobi Tabi
Leg: Hero's Shoes
Skill 1: Soul Gauge Damage A (recommended)
Skill 2: Start Dash B (personal)
Skill 3: Nullify Ring Out C (personal)
Skill 4: N/A

While one person is working on Soul Crushes, the other player should select Ivy, since she defaults with Auto Impact A and can simultaneously work on "Like a Flowing Stream". If the other person already has this achievement, character does not matter.

The match settings don't particularly matter; with 1 round, you can quickly work towards getting "Engraved into History", whereas with multiple rounds, you can get more Impacts, Soul Crushes, and Critical Finishes with less loading time. I recommend the level Phantom Pavilion due to having four walls, preventing ring outs.

Have the other player (Ivy if working on Impacts, anybody if not) continuously guard while Yun-Seong uses his X+X combo (against Ivy for Impacts) or his Forward+Y move. Eventually, the guarding player will have a flashing red meter. At this point, disregard the X+X combo for Impacts and use Forward+Y for sure. After two or three hits, you will obtain a Soul Crush. I highly recommend capitalizing and pressing LB to perform a Critical Finish.