Assassin CC
-Electrocute: 4 second hard stun
-Spike: 2 second hard stun (can only be used from stealth unless speced into Hand Of Darkness)
-Whirlwind: 8 second mez with a 2 second cast, breaks on damage.
-Mind trap: 8 second mez, only usable from stealt, breaks on damage.
-Low Slash: A specced 4 second mez, weapon damage which is subjective to defenses, breaks on damage.
So there are two (2) stuns the assassin has which do not break on damage, the rest do. As far as killing someone in 4-5 seconds (1.5 gcd would land just two hits) is next to impossible unless the stars align on a terribly unfortunate sorc. As far as stunning someone for 8 seconds, yes almost all classes can do this with a mez and for any duration longer than that (IE another form of CC like a hard stun or mez) will result in a full white bar.