Hiruzen Mastery of Countless Jutsu

ShounenSuki 

「火影様とは仁・義・礼・智・忠・信・考・悌の理を知り」
「千以上の術を使いこなせてはじめて...　ん?」

"Hokage-sama towa Jin, Gi, Rei, Chi, Chū, Shin, Kō, Tei no kotowari o shiri"
"Sen ijō no jutsu o tsukai konasete hajimete... n?"

"The Hokage-sama knows the fundamental principles of benevolence, justice, gratitude, wisdom, loyalty, faith, prudence, and filial piety."
"To begin with, he can easily use over a thousand techniques and... Eh?"

Viz, "You're the one nicknamed "Professor" because you knew and could perform all existing Konoha jutsu.......please don't disappoint me."

Originally Posted By NJT:

扱う = able to use / perform

木ノ葉に存在する術のすべてを知り　扱う
He not only knows but can also use/perform all the jutsu that exist within Konoha. (doesn't mean every jutsu ever created, just the ones that reside in Konoha)

In other words, if you were to add in "counter" that'd be adding a tad bit too much that isn't in there originally.

Viz, Mifune: "That's five natures all at once!"
(How do we counter that?!)

Hiruzen: "I see that!"

[...]

Swirly: "To emit the same jutsu simultaneously and cancel them out, wow..."

Kiba: "Heh heh... Don't you underestimate Lord Third!"