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 knock out the Impacts first should select Ivy, since her default character already has Auto Impact A, while the other player selects the customized Yun-Seong I recommend in my "Mad Destroyer" and "Death on the Battlefield" solutions. If you're too lazy to look at those solutions, create this 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
With this Yun-Seong, the player that is not using Ivy for Impacts can work on Soul Crushes and Critical Finishes at the same time.
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.
Simply guard with Ivy while Yun-Seong attacks high with his X+X combo. You should get anywhere from 5 to 15 Impacts per round, depending on how lucky you are.