The best way to get the Gears 1 Singleplayer achievements is to play on Co-op as Dom and on Insane difficulty. The achievements are stacked, so playing on insane nets you casual and hardcore achieves.
ok thanks man, answered all of my question plus an extra bonus
No problem. Playing the way I advised would get you every single player achievement if you beat the game, it's around 600 points in total.