Iban You from Hurting Me! is an achievement that requires the player to kill a disciple of Iban while using Iban's staff.
If the player no longer has Iban's staff after the quest, then a disciple of Iban must be killed and he will drop a broken Iban's staff. The broken staff can be repaired by the dark mage (east of the Underground Pass entrance in West Ardougne by a house) for 200,000 coins.
The staff only needs to be equipped when the disciple dies, it does not have to be used to kill the disciple. Attacking with Iban's staff can be done by casting a regular spell, or by using the Weapon Special attack ability located in the Constitution book.