You could try to search for this ^^ has been asked and answered dosens of times.
At level 10-14 you will do 0-1 dmg. Level 15-19 = 0-2 dmg. Level 20-24 = 0-3 dmg. etc....
You must consider that required weapon level minus your own level must be higher than or equal to the atk of the weapon.
Eg. Royal axe on a level 25: Req lvl: 75, Atk: 47. Counting if it will work: 75-20 = 50. 50 > 47 = it will work and you would make 0-4 dmg on a mad sheep.
The royal axe would work untill level 28. (75-28 = 47, higher level would have 1 atk on the weapon and deal dmg according to the skill formula)
@
Bashy
I would recommend going for level 14 or even more to be less suspicious to the mad sheep killers

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