There's 2 possible routes you could possibly take.
1) 40 or 50 (lol) DEX, power stance two blacksteel +10, enough FAI to cast whatever miracles you like for long range annoyance. Or:
2) Use spices to lower the requirements for sunlight blade as low as you can, get at least one lightning blacksteel +10 and use sunlight blade to buff that. Take note that you can't buff both blacksteels at the same time. Only have enough FAI to cast your lowered sunlight buff. Then you can plug whatever stats you like elsewhere.
DEX isn't really a good scaling stat. 30 dex gives 98 DEX BNS. 18 int gives the same.
Further, Blacksteel +10 is rather good, but Lightning Blacksteel +10 isn't necessarily that good. Normally, Lightning splits the damage down the half, wheres with this, it's not an even split. That's OK, but AR isn't raised from infusing it
At 40 Dex and when upgraded to +10, this weapon has an attack rating of 365(190+175).
When infused with Lightning at 40 Dexterity, and Faith this weapon has an Attack Rating 363=(Physical 172+69)+(Magic 93+29).
Normally, you'd get a much higher AR with an infused weapon, which is needed because it'll be going through two defenses. Two damage types but the same AR will always mean less damage. Always. Lest the enemy has 0 defense. Players don't have 0 defense even nude. Since it is my understanding that Lightning Weapon adds a percentage of your lightning damage to your weapon (in addition to a static 50 damage), you'll get less from the spell when you have less lightning damage to begin with.
So it is my feeling that pure dex build isn't that viable in this game, unless you can use the bleed proc to up your damage output. The scaling bonus is lower with DEX than with other stats.
EDIT: Also note that the Blacksteel katana says S on DEX scaling even when infused. That's not true. It is closer to C.