A pistol thief have 3 utilities to use for stealth, but I highly doubt they are on short cooldown enough for him to spam stealth with.
Also, If i ever see a dual pistol thief killing mesmer and guardian in a 2vs1 situation, im gonna be traumatized for the rest of my life. Guardian and mesmer is 2 of the hardest counters against thief ever.
You most likely fought a Pistol/dagger thief, who can go stealth every 4 seconds and use sneak attack, which looks like Unload for people who dont play thief
Edit: There isnt much you can do to counter it, except become a better player.
Thief.
(edited by Rissou.7213)