TLDR: Sky Splitter's buffs scale off of enemy level.
In Staff Melee, you can use Sky Splitter when you have three stacks of one of the forms of Staff Mastery to get a buff. The strength of the buff changes depending on what level the enemy is. For example, three levels of Perfection body normally gives around 9% resistance to everything. If you are level 50 and use it against a really low level enemy, then you will get around 58% to everything. This is a problem when fighting higher level enemies. If you are level 50 and use it against a level 54 enemy, then you will get only around 4% resistance. This punishes Staff Melee characters when playing harder content.
