I was a relative latecomer to the ME trilogy, starting to play ME1 after the release of ME3, but before the release of the last two DLCs. They are perhaps the games that I've replayed the most from my library. I finished ME3 in insanity with a caveat, as infiltrator I used a lot of "tricks" (strategies?) to complete the game. Possibly I wouldn't have completed the game at that level based solely on "skill". For example (warning: open spoilers!)
1) In the last battle against the Brutes in Priority:Tuchanka I didn't kill all of them between the insertion point and the bells (Maw hammers) to call Kalros. I just stealthed and ran toward the bells calling the maw and completing the mission.
2) In Priority: Eden Prime waiting for the pod to activate I didn't hunkered where the pod was, I ran and climbed to the roof close by and sniped all the Cerberus troops from afar.
3) Grissom Academy: I didn't walk directly into the courtyard after the encounter with Jack. I remained inside the building taunting the Cerberus' troops to come for me so I could dispatch the bulk of them one by one.
In general, I used a specific strategy for each mission that put me in an advantageous situation, not needing that much "raw" skill to complete it.