On the third mission you need to complete both objectives to unlock a skin.
1) Kill a scientist with an exploding Cow.
Pick one up and fire it in the direction of a scientist, they are marked targets so shouldn't be hard to miss.
2) Scan humans (four) at the rally
Once you have the Mayor's holobob disguise, go to the right of the stage and use
to scan four memories of humans standing infront of the stage. Once complete, head to thw stage and finish the mission.
Back on the Mothership, head over to the last page using
and then clicl on skins. Crypto's Moo-vasion skin has unlocked, equip it to gain the trophy.