All According to Plan...
From Halopedia, the Halo wiki
All According to Plan...
All According to Plan... is an achievement in Halo: Combat Evolved Anniversary that is awarded for killing all the enemies in the first encounter in the level The Truth and Reconciliation without being detected. It is worth 25 Gamerscore points.[1] It is represented by an M6D target reticule surrounded by a pink star, which in turn is surrounded by a purple octagon.
To get this achievenment, it's a good idea to snipe from behind the rocks patiently, taking cover whenever enemies start to run blindly towards the player. When no more enemies can be seen, walking along the ledge would usually reveal a sleeping Grunt that may be on the lower level.
The Achievement is worth 25 Gamerpoints.
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