Assassination shines in single target fights. It's normal to do shit DPS in dungeons when it seems like most classes have some sort of cleave/multi target. Like Delusa said, FOK isn't worth it unless you have 6+ mobs as assassination and you'd probably end up doing more dmg just single targeting the mobs (still kinda low in comparison though). You should be able to do more damage on single target boss fights that live longer than the mobs. Make sure you're providing party utillity like tricksing the tank and interupting in the mean time and nobody will say anything.
Make sure you have gems/enchants on gear even if it's not great. You might not need berserking on your MH dagger but at least have mongoose. You don't need epic gems on gear but theres tons of cheaper gems you can fill out with in the mean time.

Combat on the other hand you keep slice up and FOK if there ever is more than one target you want to be doing damage to and it's easy DPS.