Age of Wonders: Planetfall

The Infiltrator's Holoshift ability allows them transform into the enemy target as well as copying it's attacks and abilities, so it's a fairly straightforward achievement. However it's worth noting that the Infiltrator only needs to get the killing blow. You can use another unit to weaken the target before finishing them off to make things a little easier.

Age of Wonders: Planetfall (PC) - Achievement - The superior version

While holoshifted, use the Infiltrator to kill the unit it transformed into

This achievement is fairly self-explanatory. Simply use the Shakarn Infiltrator to transform in to an enemy unit and then get the killing blow on it in a manual battle.