Over at Rooster Teeth,  Milesprower has put up a video  showing how to get the first three target marker achievements. The vid says it’s only for the original on 360, and not Colonies, however, this video WILL work for Colonies since the markers are in the same locations. For that matter, you could also use this for the PC and PS3 versions as well. Also be aware that he is playing on EASY, as the target markers for Normal, Hard, and Extreme are in completely different locations. He also says you have to make it through on one life, but that’s not entirely true… If you get a few markers then make it to a check point, you keep the markers you got after you die. So for instance, if you make it all the way to the boss, you don’t need to go back and get the markers. Just beat the boss and get the points! If you’re looking for a few rather simple achievement points, or want to get closer to unlocking Luka in the PS3 version, this video will certainly help you out. 

Just another tip to add to this; if you’ve already gotten some of the target markers in a previous play through, you don’t need to go back and get them again. You only need the markers you have not gotten. Also, if you are playing on Easy, you can use all the cheats (like infinite health) and still unlock the achievements, or Luka on PS3. 

If you need the other difficulties, or just the other missions on easy, here’s a guide to EVERY marker . It’s translated Japanese, and simple pictures, so it’s no where near as helpful as Mileprower’s video, but it’ll hold you off until he puts out a guide to the other markers . ^_^