Well they have pretty solid attacks and combine that with high strenght- then they are like Besererkers but they have a low fighting skill and not very remarkabel defence or courage either and the defence is avearage...
The "Elven cloack" makes them like stelath units and can provide some cool tactical possibilities (you know like the swat team or something)
But IMO they are overpriced-> the elven cloack doesn't compensate the fact that they can get easely piced off in a fight...
But if Id use them, then i would use about 3-9, that should be enough but I ten to use Giant spiders instead