During the campaign you will be set trials on every level of every mission. These are secondary and are not required to complete the mission. These usually include trials along the lines of completing certain areas of missions while remaining unseen by enemies and by achieving high accuracy scores. To unlock this achievement, you will need to complete all of the level trials for a single level of your choosing. It would be easiest to attempt this achievement during the first level of the first mission 'Goldfinger - Auric Enterprises'.
There are 3 level trials for each level of each mission, 1 for each difficulty. For the first level of the first mission 'Goldfinger - Auric Enterprises', the level trial for the operative difficulty is 'Accuracy' and the goal is to keep your weapon accuracy above 90%. Take your time when lining up shots and try to use single shot weapons, especially your pistol.
The level trial for the Agent difficulty of the first level of the first mission is 'Target'. For this level trial you need to shoot 7 wooden targets dotted around the level.
The level trial for the 007 difficulty of the first level of the first mission is 'Stealth'. For this you need to make it from the beginning of the smelting room to Goldfinger's office without raising an alarm. A video walkthrough of this trial can be found below.
All of the level trials are difficulty specific, but do count if you're playing on a higher difficulty than is required for the trial. So you can do all 3 at once if you play on the 007 difficulty, making this task harder, but saving you time.