Improved Skirmish

 
Prerequisite(s): Character Level 21+, Skirmish +4d6/+4AC or better
 
Benefit: If the bonus to AC granted when you use your skirmish ability is equal to or higher than the number of dice of extra damage granted by your skirmish ability, your extra damage on skirmish attacks increase by 1d6. If the bonus to AC granted by your skirmish ability is less than the number of dice of extra damage granted by your skirmish ability, the bonus to AC when skirmishing increases by one instead. This feat can be taken multiple times, it’s effects stack.
 
Source: Complete Adventurer