You'd have to be level 7 Cleric (if you go full Cleric)
From the SRD
Improved Critical [General]
Choose one type of weapon.
Prerequisite
Proficient with weapon, base attack bonus +8.
Benefit
When using the weapon you selected, your threat range is doubled.
Special
You can gain Improved Critical multiple times. The effects do not stack. Each time you take the feat, it applies to a new type of weapon.
This effect doesn’t stack with any other effect that expands the threat range of a weapon.
A fighter may select Improved Critical as one of his fighter bonus feats.
You need to be a 11th level cleric to have BAB +8 and you dont get a feat before to use before 12th level. So a 12th level cleric can qualify.