Step #3: S Ranks
Most of the levels are straightforward and S Ranks are no problem but there are no specific requirements as to what an S Rank looks like. You are measured on Time Taken, Kills and Damage but never told how they are impacting on your overall score. Completing harder Challenges without any damage is impossible, but without any idea how much is acceptable there's no way to adjust your play other than trial and error.
There are also several Challenges that are open-ended, free-roam levels. These have absolutely no indication of what you should be doing, let alone how to S Rank. Add to this the need to play through the whole game again as levels to S Rank each one, and you've got one of the toughest and most time-consuming achievements I've come across.
The S Rank Guide
Clash Of The Titans isn't going to win any awards, nor does it provide the usual 1000-points-for-free normally associated with movie tie-ins. I personally enjoyed the challenge, and found the combat system, fighting, and weapon levelling rather engaging. You need time on your side though - 15 hours for story - 45 hours to collect everything and upgrade it - 60 hours for the S Ranks