First you walk into the place where you are going to fight. Get over to your allies. Once they start running collect whay tou need and follow them. First three portals land. Use all of your ammunition to destroy them. Don't save anything for the final boss. Then a huge piece of ^__^ lands and a troll comes out of it. Kill him and get into the shot down Sleg ship. Minigun rounds appear there. Keep shooting the final boss in his feet until he dies. Don't come out or else he'll tear you in half i n no time. After that relax and watch the final ending scene.