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):
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.