If you want to be able to reliably hit CR 15 monsters, you'll want to be able to hit AC 30 with a roll of 9 or higher (I prefer the 60%+ chance to hit), so you want to swing at least a +21 with each attack.
For your go-to spell, I'd recommend Divine Power as its a combination of Divine Favor, haste, and a False Life spell. You can pop it as a swift action with Fervor.
Are you a strength based Warpriest or dexterity based one? If you are strength based, you'll need at least 15 dexterity to get that TWF feat, though dex based costs an extra feat for weapon finesse.
Anyway, you want your main attack stat to be at least 22 at 12th level (or higher).
BAB 9
Str/Dex +6
Weapon Focus +1
Sacred Weapon +3
Existing weapon enhancement +2
Divine Power +4
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There is a +25 to hit right there (-2 for TWF)
You don't NEED to be running that all the time, but that will get you where you want to be.
Going with the Arsenal Chaplain archetype, and Weapon Finesse for Dexterity to attack, you can take the Trained Grace AWT to double your Strength modifier for damage.
Possessed Hand gets you a +1 insight bonus to attack and damage with any light weapon used in that hand... Hand's Automony will reduce TWF penalties by 2 to a minimum of -1... masterwork/Weapon Focus eats that remaining penalty pretty easily. Hand's Detachment even gets you a Familiar. Lol.
As divine favor and divine power grant luck bonuses, don't forget the Fate's Favored trait.
Off the top of my head:
1) TWF, weapon finesse
3) double slice, pirahnna strike
5) AWT: pick one to taste
6) weapon specialization, lunge,
7) open
9) ITWF, greater weapon focus
11) another AWT
12) critical focus, open