It takes 4-5 runs to S-Rank ME1 so here's what to do to get it ASAP...
- Run 1: Soldier class, play paragon or renegade, take the first two squad-members and play with them for the rest of the game, unlock all weapon based achievements, beat the game once.
- Run 2: Adept class, play opposite attitude of what you did with soldier, take second set of squad-members, unlock all adept power based achievements
- Run 3: Engineer class, play whatever attitude you wish for this character, take last two squad-members, unlock all tech power achievements
- Run 4: Take your favorite class for the hardcore achievement run and to unlock insanity
- Final run: Beat Insanity
The reason why you only play as Soldier, Adept, and Engineer is cause they have all the powers/strengths of their class. Infiltrators don't have AI Hacking like the Engineer and the achievement only counts if your characters delivers the blow. You can dumb the number of runs down to 4 by playing on Hardcore while on your second or third run, but you'll be reminded about how under-leveled you are.
Whatever the choice I'm wishing you good luck in your endeavor. The ME franchise is a must in my book even if you don't think the genre and style fits your taste. It's a must down the line in anyone's backlog. Good luck, and have Batman. =]