Stalker: Spear wielders with the Rogue's poison enchantment, they can be very dangerous at low poison resistance levels.
Tracker: These are basically archers with no special skills. They are very weak and shouldn't be a problem, not even for low-leveled players.
Scout: A variant of Trackers. They use Bow and Arrows as their main weapon.
Huntresses: Archers with traps from the Rogue mastery.
Alchemists: Weaker sorceresses, they rely mostly on their powerful poison but can also use lightning strikes. They usually use their Vials of Posion, when you are far from them, which explodes and emits deadly poison and pierce damage.
Sorceresses: More dangerous than the alchemists, as students of storm, Sorceresses can call down lightning bolts that can sometimes instantly kill. High amount of lightning resistance is recommended.