It's a longass formula and I suck at formatting posts. Prepare yourself.

Character Attack x (1+assault+charaAtkEid+attackBuff+assist) x (1+elementMult+elemAtkEid+elemBuff) x (1+attackDebuff) x (1+unionBuff) x (1+otherBuff) / (enemyDef x (1+defMod))

Character Attack is listed on the edit page.

assault is 6 per SSR assault, 3 per SR assault, 0.5 per level of assault/pride. Add those 3 numbers up and divide by 100.
charaAtkEid is the sum of your main eidolon and support eidolon, divided by 100.

attackBuff is listed here: http://xn--hckqz0e9cygq471ahu9b.xn--...9E%9C#ab98jh65

assist is the passive skill of the soul/kamihime. You'll need to check your kamihime's passive and find the amount on the wiki.

elementMult is -25 if the target resists your attack, 45 if it's weak and you are using fire/wind/water/thunder, 48 if it's weak and you are using light/dark. 0 if neutral.
elemAtkEid is the sum of your main eidolon and support eidolon, divided by 100.

elemBuff is listed here: http://xn--hckqz0e9cygq471ahu9b.xn--...9E%9C#e10370d0

attackDebuff varies per enemy. Obviously not important if the enemy doesn't debuff your attack.

unionBuff is the effect of your union's Guardian. 0.02 to 0.06
otherBuff is... I don't know what otherBuff is. Sorry.

enemyDef is usually around 10. Just use 10 if you don't know.
defMod is the sum of the target's def buffs and def debuffs. On an unbuffed target with Sniper Shot and Ambush active, -0.4