I think that berserkers are too overpowered. One hit - person is laying on the ground. After several seconds - death. I think that berserker should not put you on the ground so often or if I'm laying on the ground dmg should be lower. Now usually wins a team with more berserkers. I'm playing for berserker myself but I wouldn't say anything if I would do less damage.

The left ctrl button might change that on the ground thing for you

Block.
Learn their animations. Their block breaker / knock down is incredibly telegraphed and slow to use so get out of the way.
Basically, learn to play the game before you start crying about class imbalances.

Easy solution, dont end up on the ground...... posters above have given you some tips on how you could do that.

If I get finished on the ground I dont blame the zerk being OP, I blame myself.

Well, wait for higher levels, you end up on the ground really easy, since Tanks have 2 spells which are hard to block and bang, you on ground being executed by zerger.
Or if you somehow manage to stay on feet, woalaaa crushing blow hits you, even with block on you end up with like 1/4 of you HP and anyone can say anything, avoiding that OP spell, especially when zerger has attack speed amulet, is nearly impossible.
If anything should be nerfed on that class it is definetaly that spell. On all other OP looking spells you will soon or later get used to.

What role are you playing? If it is a melee class you should be blocking by default, if I am in combat I block unless I am doing something else. Always look to counter attack not start the attack if you are struggling.

you cannot block crushing blow...

You can't block the damages but you can block the knock back effect.

You didnt answer my question Enigma, what role are you playing? The only roles which should be regularly coming into contact with zerks and tanks are other zerks and tanks. Other roles should be picking the times they engage, casting / attacking and getting out. I know about higher levels, I have four characters playing both sides of this vrs lvl 20's. When I play my tank or zerk whether or not you I can catch casters, healers etc with knockdown etc is total down to player skill not game mechanics.

EDIT - I would also point out that at early levels Crushing Blow costs 32 CPS's (still 21 CP's when at level 4) and finishing blow costs 15 CP's (10 at level 2) so it is not something zerks can just use all the time. Like the other tank skills that causes knockback - battering ram which costs 45 CP's even at level 2!

All classes got 1 hit = 1 kill the difference it's making by player skill .