That's called a UN Security Council resolution. It's the only way to do that legally. Both China and Russia have continued to basically block that and veto it.
In answer to your previous question, in fact if you look at the manpower of the Syrian army versus the collective opposition—the Free Syrian Army, ISIS, and Islamic Front—the difference is not that much. In fact they are almost equal in terms of what's been left of the Syrian army, because so many defectors have left, if you will.
The challenge, as you said, is the guns. Syria does not only have replenishment through much of the legal channels via the Russians but they also have air superiority, which has become the most devastating thing. They're not actually sending troops to go in battle anymore, because they don't even trust their own troops. They're just basically bombarding things from the sky and they are getting replenished constantly.
The other thing too is that where the Syrian army has been supported by both Hezbollah and by the Iranian Revolutionary Guard, those are two militias that actually have a lot of background and history. In other words, they're organized and effective fighters. If you look at the Free Syrian Army, however, they are already ragtag to begin with. They're defectors from the Syrian army. Then you add ISIS and Islamic Front, which are basically based on volunteers. Yes, the volunteers are coming from all across the world, and in some cases through the region, but they are not necessarily a real professional army. Their tactics and logistics are also hampering their effectiveness.