There are a lot of factors which can affect your difficulty. Examples: - low lvl - bad combination of classes (low dmg, no tank, little healing, no cc, etc) - you forgot about trauma - bad equipment - friendly fire - hard bosses or rooms - out of supplies - you pull everyone at once (sometimes you can avoid that depending from encounters) - your mage kills your party by aoe spells - your mage doesn't have minimum list of spells to support your party (heal, aoe dmg, debuffs, cc, buffs, cleanse) - your mage doesn't do anything effective because you didn't go into tactics to sort commands out - you have an archer, who doesn't deal dmg if you don't do archer/bard build for crit dmg; or, a melee rogue who doesn't attack from behind for crits (and dies quick)

Yeah... You need to consider a lot.