copied from "JackBlaise" on trueachievements.com
1. First of all make sure that you unlock the achievement Bushido and Chill. There is a great guide on TA (see link at bottom).
This should be completed as soon as possible as it must be completed before the point of no return (Mission: Nocturne Op55N1). Just to note as well, you SHOULD read all ending guides in advance in order to make sure you don't miss anything.
2. Make your way through the quest Nocturne Op55N1 until you are on the rooftop with Misty. ANY dialogue here will work.
3. When talking to Johnny on the same roof, you MUST select "Think you and Rogue should go."
4. After selecting this, you MUST take the pseudoendotrizine.
5. Keep going through the mission until you defeat Adam Smasher get to Mikoshi. There is a conversation with Alt and V (and yourself, Johnny) whereby you must select that you (Johnny) go with Alt across the Bridge.
6. Cross the bridge (V will try to stop you but ignore him and do not agree to let Johnny keep his body) and at the end select to cross the bridge.
7. If done correctly, the mission Path of Glory will start. Complete this and when the credits start your achievement will unlock.