It would be difficult to overthrown Assad with the kind of support the receives from Russia (provision of air support and diplomatic cover), Iran and support from several shiite militia this is almost an impossible feat to overthrown Assad in the nearest future. All these groups support Assad for their own personal benefits.
Keep in mind that overthrowing Assad would cause a lot of catastrophic events in Syria similar to what is happening in Libya. If Assad is removed, Syria would become another home for extremists like ISIS.
The best way to solve the ongoing crisis in Syria is to get all Syrian rebels together and get them to cooperate with the Government and work towards the stabilization of the country. After this, issues like Assad leaving power can be discussed.
Assad has become a symbol of resistance against the American hegemonic designs in the Middle East as well as a representative of Russian interests in the region. His removal from the scene without Russian consent is impossible without serious repercussions. There are three possibilities to throw Assad out of power. These include a coup or assassination by the armed forces, aerial bombing by the NA to forces for which they want a No Fly Zone as they did in Libya.
This would result in a full-scale war between the two Russia/China/ Iran/Nexus and the NATO/Israel/Saudi Arabia axis, a friendly sell-off by Russia whereby NATO assures Russia of the protection of its vital interests in return for the voluntary abdication of Assad to St. Petersburg.