Strictly speaking, that requirement is implied for stinger, because stinger is described as 'zokusei kyarakuta no kyuusho kougeki kakuritsu UP'. Zokusei kyarakuta is just the 'character of X element' part we see in every weapon skill. Kakuritsu's probability. 'kyuusho kougeki' is what we also see in abilities we refer to as crits, like Asherah's.