Which level can you hit 15 with rune scimitar

With no other equipment bonuses (such as an Amulet of Strength or Gauntlets), you can hit a 15 on the Aggressive style at level 71 Strength, or 74 Strength with an Accurate style.

With a more reasonable bonus, such as an Amulet of Strength, you can hit a 15 at around 65 Strength on the Aggressive style, or 67 on an Accurate style. Other equipment may provide a Strength bonus, such as a Rune Berserker shield, so there is no single answer as to what stats you need to hit a certain number.