Ps3 – How to someone help me beat a Boss in Dark Souls Co-Op

From (one of) the Dark Souls wiki:

• Humanity can be earned by playing as a Phantom, and help kill the end-level boss.
• Humanity can be earned by killing Evil Spirits (invading players), or by invading and killing a player in another world.
• Humanity can be bought from some merchants.
• Humanity can be found, as a dropped item, from some killed enemies.
• Humanity can be found as a consumable item.
• Humanity can be earned by having your hint sign up-voted by another player.
• Humanity can be earned by killing a large amount of monsters in one area if the boss is alive (up to 10 Humanity).

The last reason must be why I was gaining humanity when spawning.

Since no one answered and I managed to solve the issue myself, I'll post my solution.

I replayed each level from the last Greece level till the end of the game by choosing Replay Level from the menu and picking the hard difficulty. After completing the final boss the Tough Enough trophy unlocked.

What happens, according to to other people's experience is that if during your hard story mode playthrough you replay a level or do a time trial, your auto save lowers its difficulty to Medium, you have no way to know that but by looking at the difficulty level the game offers you when choosing to replay a level after you've loaded your main save (it offers the last played difficulty). Since I knew where my game changed difficulty, I was able to replay only the levels I messed.

I hope my experience helps others with this issue.