Ranger Difficulty: very easy
Hunter Difficulty: medium
Short to medium ranged moderate sustained damage dealer. Reduced damage in close range.
Warrior Difficulty: easy
Short ranged high sustained damage dealer.
Rogue Difficulty: medium
Quick, fragile, high burst damage. Primarily melee with strong close-quarter attacks with a few gap closer skills and short duration stealth. (Guild Wars 2 style thief)
Elementalist Difficulty: hard
Short to medium ranged enchantment-based attacks dealing moderate damage. (Think Legend of Korra)