Troll regeneration has two facets in 4e: regen hp, and coming back from below 0 hp.
The first is the real big issue. Unless you're able to do fire damage every round, it will regenerate. Plausible if you have a wizard, say, but how often do you fight only one troll? Other classes can, of course, do fire damage by wielding torches or what have you, but that's a lot less damage.
Once a troll is down, keeping it down is easy, just do a little fire or acid damage and it's over. (No need to, as in 3.x, to do its full hit points in fire or acid damage!) However, that takes an action which you might use to engage another troll that's about to eat a squishy.