You can always crank up the difficulty if the game seems easy. But don't go too high or the killing speed slows down which results in less exp/hour.
Here’s how the lower difficulty levels increase the basic monster stats and exp returns:
Hard: 200% Health, 130% Damage, 75% extra gold bonus, 75% extra XP bonus
Expert: 320% Health, 189% Damage, 100% extra gold bonus, 100% extra XP bonus
Master: 512% Health, 273% Damage, 200% extra gold bonus, 200% extra XP bonus
(shameless copypaste from
Incgamers)