Poor Defense Tactics
Waiting for the soldiers to come to them guaranteed failure. All they would have had to do was station natives at the top of the falls. Put 20-30 all around the various rises and they could have held off the soldiers forever. Cannon fire at them would have done little and the soldiers would have quickly run out of shot and powder. All the natives would have had to do was tumble a few rocks down on anyone who tried to climb.
While we're at it, since they took powder and weapons from the soldiers at night, they were obviously going to use them to kill soldiers, so again why wait for them to come to you. It's not like it's more religious to kill the soldiers later rather than sooner. Just slit all their throats in the middle of the night, or just set the powder to explode while they are all sleeping.
That was a badly written section of the film.
"My name is Paikea Apirana, and I come from a long line of chiefs stretching all the way back to the Whale Rider."