I'm still sub-50 though not for much longer and my experience in pvp might have given me the wrong impression because I dispatch anyone who challeges me. Not as surely as I did on my KC shadow but still.

while i hate talking about 1v1 in any pvp situation. i understand it happens. As an assault spec vanguard. i can 1v1 any class. the hardest would be Sentinel/Marauder. still possible, but you have to time out for defensive cooldowns and stuns. it's not like sub-50 pvp. in endgame. people know how to play their class.

The only classes you really have to worry about is Mara and Sniper.

Assuming equal gear and skill you will lose to:

 

Sentinel - He will have ~5% health left. Every time. If you're allowed to kite (move outside the duel area) you may be able to win.

Scrapper Scoundrel - If he is allowed to open. Good ones will vanish when you pop your shield and CC you so you can't med. You have a chance to counter with scan.

Troll Spec Shadows (23/1/17) or Tank Spec Shadows (31/10-x/x) - Again the key to them winning is CCing you or disappearing while your Shield is active.

 

In a non-duel (someone gets the jump on you) situation, you can easily die to snipers, juggernauts or even sorcs. You should not be losing to any class if you are getting the jump on them, barring sentinel with all cooldowns available.