The scarab-man (who you can actually see trailing you at several points, you can't shoot at him and he'll disappear if you get close) will give away the rebel position and you'll be forced to ride sidegun. Unlike in MGS3, only pistols, SMGs, and grenades can be used in this fight. Petro bombs, if you have any left, are good because they detonate on impact. Look for exploding barrels and shoot at them (if you're not going non-lethal, that is).

To fight the Gekkos, empty clips from the SMG into their legs and don't worry about them after that. When the aerial vehicles show up, use the SMG to rake them before they launch their missiles. FROGs will start jumping onto the van, so shooting them off should be your first priority (I don't know if tranquilizing them counts as a non-kill since they fall off). Throw grenades when the enemy blockades your way; after that, there'll be FROG troopers running through the streets attacking you and the van.

After a cutscene, you'll have to deal with Raging Raven's unmanned aerial vehicles. Aim for their wings and keep up a hail of fire; it's difficult with the Skorpion, since it goes through bullets so fast that you have to keep reloading. However, keeping up a defensive screen of bullets is your only real option here, so a non-SMG won't cut it. Keep going. Enemy vehicles will show up, as well as FROG units, Deal with them like before. In order of priority, it's UAVS - vehicles - FROGs. You'll end up in an alleyway, so watch out for explosive barrels, which will hurt you now as well. Eventually you'll end up at the next area.

Metal Gear Solid 4 Walkthrough