Never tutored Calculus (yet) but aced three semesters of it and used to "lead" study groups with my friends.

I find that what fucked fellow students in Calculus most often was not understanding logarithms. Not knowing how to handle something like

(sin(x))^(x)
limit as x -> ∞ 

absolutely WILL fuck you. My professor often went over examples like this that may not be immediately obvious, but no one ever prepared for them on the test "I'll just get everything else right"

Go through your books section on logarithms and logarithm laws and it will help you be prepared for tough questions. Not understanding more "advanced" algebra will also fuck you. You should be able to see something like

x/(x - 1) 

And know that you can rewrite it as

x/(x - 1) = ((x - 1)/(x - 1)) + 1/(x - 1)