While I know absolutely nothing about your build, you can probably kill that berserker much more easily than he can kill you. Remember, they lack the ability to use healers, to word of recall/teleport out and if their mv runs out, they are forced to sit there. Combine with plague for the win. Damage reduction goes a very long way as does the general parry/dodge ability.
For a melee char, general idea is to maledict the berserker, always ready to run away if getting mauled. There's not much preventing you from trying repeatedly. Getting a stone skin/protection at the very least is heavily advised. They can't staunch the bleeding, for example. Or sustain full range of maledictions from assassins/dagger warriors typically.
For a mage, get a lot of DR and watch your hp carefully. Try to maledict if able, lure him away from village, run to the healer judiciously, go for multiple encounters. When that deathblow is for mauls, you're not going to feel so bad.
If you're on low ranks and die from a single deathblow, just avoid them. There are multiple areas that are very hard for them to get to. Eil Shaeria and Elemental Planes off of Consortium come to mind.