I'd lay a beam across the 5 or so joists that are sagging and slowly jack it up level using basement support poles that you can screw by hand every few days till the floor is level. Once level then decide what permanent supports or sistering is needed. At that length of span, consider using a steel I beam or H beam otherwise a wood beam would have its own sag.
Short of that, get at least one support pole (they are 20 some bucks at home cheapo) and put one in the middle of the sagging area and see if that brings up the others and if not then put a pole under each joist and adjust on each till level. Adjust the poles every few days and it may take a while to get it back straight. Sounds like your joists are undersized. Forget the leveling compound.