I must agree with Optimal...i'm an IG player and i wouldn't wanna be anyone else on an assinate mission. First of all we have the assasin, who does what he says on the tin. So long as you can find there command unit (tricky with eldar cos it should be in there webway if they are a decent player) he can take them out without ever being seen and from along way away, perfect! We also have the same type of advantage that eldar have in that we can put our command squad in a building so he can't be attacked or found...if they do start to attack the right building we can just move him to another without ever comin out. Meaning our entire base would have to be destroyed to get to them.
ok that is all v well but think about it like this, if u att a farseer in h2h and surrond it, then it will do the weird thing and send all units close to it flying. if u surrond a big mek he will just simply teleport away...