When I read this question, the first thing that came to mind was the Comanche tribe of Native Americans. Throughout the 18th and 19th centuries, the Comanche fought against settlers encroaching on their lands (the southern part of the North American Great Plains). The Comanche war bands weren't very far past a stone age level, using arrows and lances fashioned using primitive hand tools. The foes they were fighting had pistols and rifles, built fortified bases, and had formal military training. Despite this large technological gap (roughly similar to what the OP is going for), the Comanche were able to successfully hold their own against a significantly more advanced foe for a very long period of time. Your ancient people can use similar tactics and methods to fight off their technologically-superior invaders. (for more details on Comanche warfare, see the excellent book Empire of the Summer Moon)
Comanches swarmed their enemies. One on one, superior firepower usually wins. Comanches attacked in groups and on horseback, and opponents had a hard time tracking so many fast-moving foes that were all moving independently. The psychological impacts of being vastly outnumbered should also not be ignored. The alien population numbers that you give are low enough that swarm warfare could be an effective tactic.
The Comanche protected themselves from gunfire using shields. Several layers of dried, tanned buffalo hide was capable of stopping a rifle shot. Even though an AK-47 round is more deadly than a 1850's rifle slug, it's plausible that an ancient civilization could develop a type of shield capable of either stopping oncoming gunfire, or at least rendering it non-lethal. The American TV show Mythbusters has done lots of experiments testing whether common objects are bulletproof, and they've shown that objects made of materials available to the ancient Greeks (ceramic, nickel, etc) were capable of stopping certain modern bullets, or at least robbing them of enough momentum to make them survivable. Layering materials like nickel and dense hides could result in a shield capable of protecting against a modern automatic rifle (provided that you're not subjecting it to sustained, focused fire). It may be heavy enough to require a dedicated shield-bearer, but I believe that was already used as a military tactic during that time period.
The Comanche also had a large advantage by being masters of horsemanship. Horses provided unmatched speed, stamina, and maneuverability in their world. Even in your world, a well-coordinated and well-trained mounted cavalry can be a threat to armored military vehicles. A tank can match a horse in speed, but not in agility. Tanks are great against massed or fortified enemies, but much less efficient at attacking a swarm of enemies that are spread out, surrounding it, and constantly moving. An agile cavalry can cause tanks to catch each other in crossfire, or deliver soldiers close enough to jump from the horse onto the tank and attack it directly. Tanks are stopped/slowed by terrain that a cavalry can successfully navigate (narrow passes, forests, etc), and even a primitive civilization can create traps to defend against tanks (such as moats or tiger traps).
Also, remember that even the ancient Greeks had access to what is still one of the most destructive weapons available: fire. Unless your aliens have advanced to the point where everything they use is metal or plastic, fire is still a real threat. A small group of soldiers making it into an alien building with torches can easily start a fire capable of destroying most of a base. Several ancient civilizations were known to use early forms of incendiary weapons, so this isn't too far from reality. Your example listed the aliens landing in Syria - a region not known for having plentiful water - which would increase the danger posed by an army of arsonists.
Any sort of a war like this would revolve around attrition. The aliens have no reinforcements and a finite level of supplies. Once they run out of certain key supplies like fuel and ammunition, it's unlikely that they'll be able to manufacture more (at least not in any real quantity). At that point, they face many of the same problems that a modern army would face if time-warped two millennia into the past. They're stuck with none of the modern trappings that they've been accustomed to for their entire life. At this point, survival is difficult even if you completely take away any sort of military threat. The aliens would have no modern medicine, little (if any) knowledge of Earth's plants and animals, no familiarity with the local geography or weather, and would likely not know how to build tools or buildings from complete scratch. If the local people could force the aliens to burn through enough of their consumable supplies, a good, old-fashioned siege could be very effective.