MONSTER HUNTER FREEDOM UNITE?????????????

I NEED SUPER HLP! I got a new urgent quest called "A STATE OF CRISIS" with a new monster I have never seen before. He's so huge, I think the back of his shell is suppose to be a lao so lung skulll. I know nothing on how to defend a ton in this game for I have never done it before. Please someone tell me how to kill the Shen GAOREN? I was told i was suppose to use some spear thingy thats on the building but it only does one attack then stop. And the village chief said for me to use CannonS but I don't know where to get some. And my archery skills don't work at all on that giant. And im pretty sure melle weapons won't work either. Tell me everything I need to know on this mission please!