Kind of ambiguous in the description above - but the special attack is a magic projectile and infusing with magic will cause the damage of the projectile to increase. However, the damage of the projectile does not seem to be particularly affected by the actual level of the users intelligence (tested at 1 intelligence and 99 intelligence, each +0 rapier shot did roughly half of the health of the Scythe wielding jailers in the Lost Bastille). Damage also seems to be unaffected by dex scaling of the weapon as well, using similar test from above. Damage is increased slightly by magic clutch ring (~20 damage at +0). Upgrading weapon does increase damage, but not a ton (seems to be the theme in the DLC weapon, with almost all weapons having high base damage with minimal improvement to be gained from weapon upgrades)
IMHO seems like the best way to utilize would be to get min stats and just stock 3 +0 magic ice rapiers in your left hand slot just to have a powerful, spammable ranged attack - Seems way cheaper than actually farming soul spears - and you can do it on any build with only 15 dex and 11 strength.