This guide is for those who cant unlock it after new light update.
There is a new mode included on the MP section. ITS GAMBIT. You can play it without the FORSAKEN DLC, but for the achievement you need have FORSAKEN DLC.
In the GAMBIT section are three modes:
For the achievement YOU NEED TO PLAY "GAMBIT", gambit prime and reckoning DOES NOT COUNT.
If you win and no cheevo you have to do:
*Delete your character.
*Create one new.
*Play the new light story to reach the tower.
*Play GAMBIT and win (its a 3 rounds based game)
*If the achievement does not pop press start, settings and select "SWITCH PROFILE".
*Select your profile again and select the same character you win, and your achievement must be pop when you load the character.
The switch profile setting work for me for these achievements too:
The In A Flash achievement in Destiny 2 worth 100 pointsComplete a Flashpoint on Earth, Titan, Nessus, or Io.