lol, i myself have played sin for many years on ep5 servers. and i can tell you this, if you cant kill some1 with their combos. you have to learn how to play them properly. they dont need a upgrade.
they dont need fatal hit or anyother skill to get a stun. they require fast thinking, combos and knowledge on when to use which skill in order to kill their targets swiftly.
a ranger/assassin has dozens of ways to kill their target, each one of them has different combos. some are more effective then others ofcourse.
you just gotta find the right combos ( not talking about chains ).
and if you think you cant kill people intime then try pagan/mage. we do good dmg, but the moment any dps class is to close to us, were dead. we cant 1 on 1 90% of the people xD.
for example
VS fighter/warrior: the moment they charge at us, most of the time were screwed, cuz of the OP dmg they do with the blackhole, spins and DS. and if that doesnt kills us. their uppercut ect will with insane dmg.
in the rare case we are about to kill them, they will use their skill to instantly restore all their HP completely. and then were screwed.
VS guardians/defenders: not a chance of winning, to much hp, unpottable stun, and good dmg. and a decent ressist bec of buff.
VS assassin/ranger: maybe a 10% chance of winning, bec of their magic defy, silence skills and mental storms mostly.
VS hunters/archers: maybe 5% chance of winning bec of their magic defy aswell, their mana drain skills, their longer ranged fast dps skills, and poison.
VS pagan/mages: a to long battle, with most likely both sides ending up in quiting the fight xD and move on
VS oracle/priest: same as with pagans/mages.
(not QQing)
just telling my opinion as to why a assassin/ranger is good enough as they are.