Don't haste. Gears of War 2 is a game focused around covering, and eliminating enemies slowly. If you've already played the first Gears of War, begin on at least Normal difficulty. Compared to the first game, the difficulty has been lowered, and on beginner the game is way too easy. It's very important, that you quickly understand the active reloading. It lowers the time required to reload and also allows some of the bullets to be more powerful. You'll get the best result by hitting the RB button right before the marker hits the first point on the scale in the upper right corner.