Edit: As long as "Pay it forward" achievement is unobtainable, BFA will be as well since it requires to obtain all other achievements in the game. I will be happy to remove this section if someone confirms that "Pay it forward" can be achieved again.
Spoiler free guide
1 - Introduction
For this achievement, you will have to unlock every other achievements in the game. It will pop up instantly after your second to last achievement. The game require at least two playthrought, but there is no missable achievements. Before I go on, just a few clarification about the ending phase of the game:
When you are about to finish the game for the first time, there is a point of no return, just before the ending battle. When you complete the game, you have the opportunity to start a new game+ from this save.
* If you refuse, you will stay in the world and you can continue side quests and collectible, but your character is caped at level 22. Please note that nothing is missable even if some collectible can be a pain to reach. So if you stay, you can have all achievements except : "Epic" (for reaching level 30), "Is there anyone else ?" (for completing the crucible, because it fully unlocks at level 25), "Bravo old chap" (he can be fought only if you finish the crucible in one sitting), and obviously "BFA". However, you can beat the final boss again and then accept to start a new game+.
* If you accept, it will erase your progress (quests, collectibles and so on) but you keep your level, your skills, your bonuses from the stone and the relic quests, and your equipment but you will start the story again from the beginning. Your character will be caped at level 30 and you will be able to complete the crucible and every achievements.
If you complete your new game+ and beat the final boss, you cannot start another new game+. However, you can beat again the final boss, complete side quests, or fight in the crucible.
If you do the side quests for relics and stones in both runs, you can stack both rewards, making your character even stronger.
You don't need to collect Vulgrim coins and arbiter's scrolls, they do not count for any achievement.
However, you need the online pass to send a gift and to access the crucible so it's necessary for some achievements. It's free if you bought the game new.
2 - What you will have to do to obtain this achievement
This is basically what you will have to do, just to give you an idea.
* Finish the game at least once in apocalyptic difficulty (it's not so hard, but don't drop the difficulty during the game)
* Start a new game + and reach level 30
* Complete the crucible (in any difficulty, but in one sitting) and then the crucible boss.
* Complete every side quests (which includes collectible quests) in a single run (video guide almost mandatory ...)
* A few easy misc achievements
3 - Walkthrough
* Step 1
Start in apocalyptic, and use the video guide for collectibles/side quests. Don't bother about crucible. Complete every side quests, and defeat the final boss.
* Step 2
Start a new game+ on easy/normal difficulty. Proceed to the main quest in order to level up to 30. Then, complete the crucible.
My method (slow but fun)
* Step 1
I started the game on normal and just finished the game with all story related achievements (and a few easy ones). Don't worry about collectibles, just enjoy this excellent game ! My character was almost level 22 when I finished the game, and I never farmed. It took me about 25 hours but I really took my time to enjoy everything.
* Step 2
I started a new game+ in apocalyptic, and I used a guide for the collectible (yeah, sue me :p). It was even easier than my previous run as I was used to the game mechanics and my character build suited me perfectly (full harbinger). I finished every side quests, cleaned misc achievements and completed the main story again.
* Step 3
I completed the crucible (still in apocalyptic, it's not very hard with life and wrath steal). It took me less than one hour. As THE Wheelman 87 mentioned, if you leave the crucible, when you enter again you have the option to continue where you left off, or to start again if you prefer.
A few tips
*You can leave a dungeon at any moment if you need to restock potions. You can also destroy environment elements such as crates, some of them contains potions.
*You can reorganize yours skills at any moment by drinking a "respec" potion. You can buy them from Vulgrim who is accessible by fast travel.
*Possessed weapons are incredibly powerful. Feed them with items that have useful abilities, such as "life steal", "wrath steal", and others that suits your character build. Indeed, when a possessed weapon level up, you can choose a new ability for it, which was attached to any of the sacrificed items. For example, you sacrifice 3 items which totalise 5 different abilities. When the possessed weapon level up, you can add one of the 5 abilities. A possessed weapon cannot have more than 4 different abilities, so choose carefully (Life steal and Wrath steal are a must but weapons that have these abilities cannot be found until end-game).
If you need any further information please ask in the comment section. Hope this helps :)