Berserk increases damage you do by 10 -30%, but also increases damage done against you by 10x that amount. So if it increases your damage 20%, you take 200% more damage.
Like I said, I don't believe that bug exists where enemy levels are misrepresented, so if you are getting destroyed by enemies 6 levels lower than you and you aren't playing nightmare then you must not fully understand the gear you are wearing and the complexities of the combat system. And you should not be playing nightmare if you don't understand those things but still want to enjoy the experience.
To really have good gear you should be crafting gear specific to your build, with the highest tier materials available. Almost nothing you can buy or find will be better if you are farming materials and searching out schematics.
But if you can be more descriptive of the kinds of enemies you are having trouble with, then maybe we can be more helpful. But the other forums are better for that as they more specifically focus on combat and strategies (both combat and building your character/gear).