Can anyone tell me how to kill Gorea because I can't kill him and I try..

Okay. Look at each of his shoulders and you will see a yellow dot on each one. Shoot those until one of his arms explodes, and then destroy the other one as quickly as possible. Both arms are constantly vulnerable to the Power Beam, but if you want to use a different weapon, look at his outside color and match it up with the corresponding beam. With both arms gone, the Seal Sphere should come out. Now he can send a long tentacle out after you, and if it touches you, it will swing you around repeatedly until it smashes you down on the floor, and then picks up numerous Trocras (those small exploding things you mentioned) and throw them at you. Shoot the Seal Sphere (it still does matter which color it is if you use another gun) until there's a small explosion. It will take back the Seal Sphere, flip back over, and grow new arms. Repeat this process two more times and you will destroy it.

Hope this helps!
Imperialist

P.S. SPOILER ALERT!!!: If you want to continue and destroy the second form of Gorea, you must shoot six panels on the wall with the corresponding beam IN CORRECT ORDER. Here's the order:
1.Yellow-- Volt Driver.
2.Green-- Battlehammer.
3.Orange-- Magmaul.
4.Blue-- Shock Coil. NOTE: you must get very close to the panel, jump at it and shoot it to activate it.
5.Purple-- Judicator.
6.Red-- Imperialist.

Destroy Gorea after doing this and you will be teleported to the next area to fight the second form of Gorea. And uess what? The "Ultamite Power" is there!! Yippiiii!!!

I have to add some specifics to that answer. If you can't remember the order that you're supposed to shoot those weird colored things, here's a hint: If you have found a bunch of the Alimbic Prophesies, numbers 2-7 will cryptically give you the order. The colors they name give away the weapon though. In Gorea part 2, when you get to the area, fall down to the bottom floor and wait for the Gunship Transmission. When you get it, go to the middle of the floor and grap the Ultimate Power, which is a Arm Cannon weapon called the Omega Cannon. It's a mortar-type weapon with unlimited ammo, but you can only fire a shot once every 3 seconds. However, it's the only weapon that can damage Gorea pt. 2. There is no specific time that you need to hit Gorea with it, but the cannon doesn't shoot far enough to hit Gorea when you're on the bottom and it's hovering around the top. There is also a glitch that once you've shot Gorea at on the top floor and fall back down to the bottom, If you switch to the Power Beam, Gorea will teleport down. There is no point in doing this because if you switch back to the Omega Cannon and shoot it, it won't do any damage.

Oh, thanks SniperX, I forgot about that. But here's something else: If you have the Omega Cannon out and then you switch to the Power Beam or Missle Launcher, it will teleport to you, and then if you get the Omega Cannon out again, it will do some wierd action and then teleport away, almost like it knows what weapon you've got out. DAMNIT, THAT'S SO FREAKING ANNOYING!!!!!!!!!!!!!!!!!!

Oh well!
Imperialist

Hey! To beat Gorea, you have to shoot both of his arms off. But you can only do that with the weapon that the color he is (eg: yellow = volt driver). But I have an easy strategy. Wait for him to turn red (right after he uses imperialist [blue]) then use your imperialist to shoot off his arms in 1 shot. Once both are off, Gorea will flip over and a gold ball thingy will be floating over him. Keep on shooting it with the imperialist until it, like, blows up. Keep on doing this until it is dead.

To fight Gorea 2 ( you must to beat the game), you have to activate Photon Avalanche. To do that, you must shoot those colorful targets on the high wall in a specific order. Here's the order:

1: yellow = volt driver
2: green = battle hammer
3: orange = magmaul
4: blue = shock coil (HINT: you must jump high to be able to hit the target)
5: purple = judicator
6: red = imperialist

I'll give you some time to fight Gorea 2 and try to figure out how to beat him.

PS: when fighting Gorea 1, just keep on moving in circles around the entire stadium and he can't hit you.

Hope this helped!
Koolstr

If you want to beat Gorea then you have to aim at the yellow part of his gun arms and shoot at it. If you want to do more damage then go for the weapon thats super effective on the gun arms. If you want to do the most damage than this is what I would do: I would wait for Gorea to use shock coil then after he uses it aim for the gun arms with your imperialist and shot at them to do extra damage. If you want to unlock form 2 then do what koolstr said but I have it memorized by heart now.

This message was by yours truly OSMONORIA