Very roughly spoken we have three bad groups: a) he Assad government, b) the alliance around al Nusra and c) ISIS. No doubt the last group is by far the worst. And we have d) the democratic opposition. Till now there have been and are two attempts at solving the situation. None will work:
1) Allying oneself with a handful of good guys without using massive forces on the ground (US) may be portrayed as morally right, but it will not succeed.
2) Churchill was prepared to ally himself with the Devil, Stalin, against Hitler. Allying oneself with a devil against the other 2 devils (as Russia does) is more effective than the US alliance with a dwarf. But without more massive force than air bombardments, it will not suffice, not least because it risks uniting the opponents, thereby indirectly strengthening ISIS.
Divide and rule is a better strategy vis a vis the Islamist groups. An obvious goal must be to isolate ISIS. At the same time foreign ground forces are an absolute necessity.
A negotiated solution can not remove, but reduce the need for more force.
1) A negotiated solution involving the democratic opposition and a gradually reformed government in Damascus combined with a foreign invasion against the two Islamist alliances.
2) A negotiated solution involving the democratic opposition and the government and the alliance around al Nusra combined with military aid against ISIS.
3) A huge foreign invasion to fight against both the government, ISIS and the the al Nusra alliance.
Of course the United States and Russia should act together. If Assad is the main point of disagreement, he must leave the scene gradually or after a certain time.