Avatar: Plausibility of the Last Battle
First, it was established early on that there exist on Pandora ground animals whose hide can deflect a round. This is crucial in the final battle scene, as, the primary means of deterrence for the landed marines would be the fired round.
Second, the primary method of transport in the air taken by the marines - are helicopters. These are notoriously easy to take down - the blades are fragile. Hit them there, and down they go.
Third, sheer numbers. In the final battle scenes, the commander of the Marine forces assembles his army in a single cafeteria. They are all lined up and easy to count. Approximately twenty in a row and about 15 rows - from a visual headcount you've got about 300 men. Which would make sense, given the fact that every man there had to be flown in from halfway across the galaxy. The Nav'i have well over 2,000 , all of them mobile.
Native armies hold advantage over forces separated by long distances from their home. And their numbers regenerate far more easily than the invading forces that have to be brought in all by ship. The Nav'i , in the same position - could drink from any flower, breathe the local air. The forces of the Nav'i were also united under a single command - and were drawn quickly to a single position to defend a single tactical target.
There was also, in the final battle - the elements of surprise, geography, superior tactics - and in no small way - scattered key weaponry deployed with expert precision.
This kind of war has been waged before. The tight formation, and singleminded tactics of an army far from home - fighting a war that they truly did not in their hearts wish to fight - and with contractors alongside adopted similiar, predictable battle formations and were - with the help of a few allies - ultimately bested by ill armed, ill trained light forces.
One of the best riffs you'll ever hear is Bill Cosby's interpretation of what the revolutionary war would be like , if it were a football game.
Ok, here comes the coin toss. Call it in the air. Heads? .. Heads it is. Ok , Americans, you get to hide behind rocks and trees and wear green. You British guys over there, are going to have to all wear red and march in a straight line.
The last battle between the native forces and invading marines followed an order of battle that would allow a very narrow window of victory to a large, lightly armed mobile defense force. Especially if their instrumentation and networking was faulty. No GPS. No radar. Poor or limited connectivity , and only line of sight weaponry would allow a lightly armed force to prevail if they hit hard.
And hit hard. They did. Ewoks, they were not.