Mission 3 is the best place for this (although again, it can be done in MP, just faster in campaign). Right at the beginning, capture the primary node as instructed, then if you look toward the main battlefield capture the node in front of you and to the right. Kill the units that attack you and then send infiltrators through the rubble along the right and under the building. There is a node that you need to capture anyway, and right next to that node is a cloaked spectre. Save before you turn the corner, have your thermal vision on, headshot, load, repeat. Very fast.