For now, let's set aside the fact that ISIS represents a theocratic socio-political ideology, not a singular, identifiable group of actors, and there's nothing you can do to 'defeat' an ideology. It would be like the late lamented 'war on terror' - you cannot fight a war against a specific tactical methodology. What we're really talking about, in this case, is finding a way to end the Syrian civil war. Because as long as the various factions are at war within Syria, NONE of those factions can be defeated. Let's look at the current status map.
First, up there in the north on the Turkish border is the Kurds. They have international access for trade, supplies and fighters, but they don't have the numbers or equipment to launch an offensive across the desert against ISIS-held communities along the Euphrates. The Sunni rebels along with al Nusra hold the area around Idlib and Aleppo, but they are constrained by the government and ISIS forces that completely surround them. They don't have direct access to resources, depending on access to Turkey through the Kurdish Kobani enclave and US/Coalition air drops.
ISIS controls the entire Euphrates river valley from Raqqa in Syria all the way to Anbar province in Iraq. They will alternately gain and lose territory on the edges, but once you get out into the desert it's not territory that they consider strategic. As long as they hold the fertile valley, they'll essentially have their Caliphate.
The Ba'ath/Alawite government is reduced to holding their ancestral stronghold in mountains and along the Mediterranean coast in the far west. But they aren't likely to be dislodged from this vestigial Syria. They have a deep water port and plenty of local and international support from their allies Russia and Iran, while Hezbollah holds down their southern flank along the Lebanese border.
The vast central portion of Syria and Iraq is a trackless desert. Nobody wants it, nobody cares about it, so ISIS can 'hold' it with minimal effort. All the action is around Aleppo, in northern Latakia province and the petroleum center of Deir-ez-Zur, which the government continues to hold onto despite a years long siege.
Now, think about this and tell me who is 'winning' and who is 'losing'. al-Assad knows he can't quit because he'll die, and he has no real incentive to quit. ISIS is under very little pressure except from the air, and that doesn't really do much to effect the strategic situation. The Kurds are in the best situation they've had in years, with a default Kurdish nation in Iraq and another in Syria, both receiving strong western support.
So there's no foreseeable end to the fighting, and there are multiple factions, none of which is strong enough to dominate nor weak enough to be eliminated, all of which are in combat with all the others. Don't even try to come up with a negotiated agreement that would satisfy all of them - it simply can't be done.
And this is why I continue to argue the US has no business being involved. There's no strategy that can do anything but prolong an endless ideological and religious holy war. There's no intervention that wouldn't increase the risk to America. 'Doing something' is a political decision, because in the toxic mix of American politics, the worst thing you can do is nothing.