You could go Ranger (or Archivist) and focus on the Hunter's Mercy spell. Guaranteed crit if it hits? Good enough for a 1st level spell. Another option for a similar effect is Surge of Fortune spell, from Complete Champion (although it is a 5th level spell).
If you can somehow get the prereq feats ( Far Shot, Point Blank Shot, Weapon Focus) for it, Deepwood Sniper 2 is a nice dip. Get that bow's critical multiplier up to x4. A dip in Cloistered Cleric can get you Weapon Focus and Knowledge Devotion.
Let's see what I can get.
Human Cloistered Cleric 1 (War, Knowledge, XXX)/ Archivist 17/Deepwood Sniper 2
1: Knowledge Devotion (Swap with Knowledge Domain), Weapon Focus (bow) (War Domain), Point Blank Shot, Far Shot
3:
6:
9:
12:
15:
18:
Choose one metamagic feat between Quicken or Persistent, pick Divine Metamagic for said feat, get as many turn attempts as you can and go to town. Divine Power will allow you to get that BAB high enough so your arrows matter. Practiced Spellcaster is another option.
Your other domain could be Law or Travel (for their respective devotion feats) or something else, if you prefer.