1. STAY TOGEATHER
SRSLY. Staying togeather results in team work. Team Work is key in most online play, especially Gears of War, so learning it now will help you in the long run. Really. I've played many games where guys just seperate and it always ends in failure. If everyone does this in a game that you are playing, just pick a guy and follow him. That should help your chances a nice deal.
2.HELP YOUR FRIENDS
If a commrade is downed, help him up. NO, do not kill this or that enemy first. Help your friends. You will die and fail. Not fun. So, priority number 1 should be to help downed comrades, especially if they are 0.001 inch away from you.
3. USE THE SHOTTIE
Using the lancer when you are 2 feet away is made of failure. The shotti is a close range weapon, it will fuck some shit up. HONSTLY. USE IT. Please.
Punching a dude in the face is easier than running around firing at nothing. Also, when using the lancer, do not try to chainsaw the same guy over and over again after it is obviously made of fail. Pull out the shotti or a pistol and smash them in the face.
But first of all, beat the game twice: on Hardcore and then Insane. This will get you a good deal of achievements and vastly improve your skill level.