While playing in the campaign, certain squad members will have a special skill meter fill up as you kill enemies and perform heroic actions (saving allied soldiers, capturing enemies, etc.). Pierson has a binoculars icon which signifies his ability to spot enemies (outlining enemies in white). When his meter is full, like all others, face him and press
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During the campaign the skills are introduced as you go on and you will find his ability available right at the beginning of Stronghold. If you want a good place to farm the 25 times to fill up the meter, do so when you finally make it to the top of the church tower (in Stronghold) where you must provide cover with a sniper rifle. After several headshots, his meter will fill to full. It is a simple place to use his skill 2-3 times and restart checkpoint and do so again very quickly.