This ability is pretty easy. In this case you have to stealth hijack about 5 tanks or APCs, which simply means you have to hijack tanks - usually ones that are already commandeered without setting off an alert. I recommend starting a little chaos to draw them near - consume one of the soldiers first - then escape and shift into that form. Once the alert is over, simply return to those vehicles and you should be able to hijack them with little problem. Once again, easy.