Bossscaling?

Hi,

why is it that i bosses scale so strange?

i (lvl 34 ranger) wanted to help my gf (lvl 14 witch) with a act 1 executioner boss. i took minutes to kill him. my dmg wasnt going through.

i play poison ranger with plaguebarrer. the plaguebarrer dindt went to 100% all fight.

it is annonying enogh that every boss turns into a pinnacle fight but at least let us help low lvl ppl.