When facing lot of magic with PA, i love rushing mageslayer after boots
-it give her possibility to take fight early, negating 2 people at the same time magic strenght with dagger and autoattack.
-it give her the mana to farm easier, and the durability to take fight earlier
-even if the damage are not huge, PA don't really damage at this moment of the game cause she got lv1 ult. So at least, you can debuff ennemy and be tankier, which result in a better fighting state
I often keep it till 6 slotted, then (if the game is too long) sell it for a better late item. I know it slow a bit his tempo, but it help a lot to not be judt an "afk farm", and make you able to fight nicely without being squishy. I'm only guardian, so i don't know if my idea is wise, but it gave me really good results from now on.
Source: https://www.reddit.com/r/DotA2/comments/s0c1ik/it_can_be_weird_but_i_love_mageslayer_on_pa/
