The Cunning Bow grants Claude +3 Speed, and as is traditional for bows has advantage against flying opponents. However, the primary effect of the Cunning Bow is an in-combat -5 debuff to all stats as long as the total visible buffs on Claude and total visible penalties on his opponent is equal to or greater than 10.
Grants Spd+3. Effective against flying foes. When total bonuses on unit + total penalties on foe ≥ 10, inflicts Atk/Spd/Def/Res-5 on foe during combat. (Example: if unit has a +5 bonus and foe has a -5 penalty, the calculated total will be 10.)