I am installing 3/4 hardwood flooring and one room is not level most of floor is about 1/4 inch and one spot is about 1/2'' off. What is the best and for me the fastest way to level floor. I was told build up with wood then use a compound that won't crack when nails are pounded through hard wood. And also read somewhere use shingles. So can someone please advise?

Believe it or not,a half inch is not really that critical if it is in the middle of the floor in a room, especially when using hard wood tongue and groove flooring. It is however imperative to maintain a level around door frames. I highly recommend using a mixable floor leaving compound that is silica based. It is good for anything out of level under half an inch.

Would a Home depot carry this compound.

Most appropriate product would be a floor leveling compound. This is a cement product, mixed up very thin, poured out like water and allowed to self level. Any thickness over 1/4" probably requires a wire mesh to prevent cracking. Nail through to subfloor. Available at home centers.