The Doctor serves two purposes. Primarly they restore the health of their teammates and themselves, but with that comes a very powerful secondary purpose; the ability to be completely reckless and go balls-to-the-walls with a melee weapon. Normally a Doctor playing the first role will score bottom but those who use their abilities to maximize carnage will benifit from 2nd or 3rd most damage. Overall, the doctor is a bit too weak combat wise and does not get enough points. Having them start with a melee weapon instead of a gun should reward them for going all out and taking risks instead of just camping inside their teammates.
The Electrician is a generalist who does average damage over a long range. Generally a pretty balanced class with plenty of offensive and defensive capabilities, but the damage upgrade buffs the turret far too much, to the point that you can easilly topscore while doing almost nothing. I don't think the range should be nerfed, but the damage the turret can do while upgraded is a little too high.
The Journalist is a glass cannon. They are capable of doing extremely high amounts of damage to lots of enemies but don't really have that much to take care of themselves. There is nothing I would improve about the Journalist, other than reducing the pulse rate on the upgraded tower to something reasonable so you can't camp inside of it.
The Mercenary is a generalist like the electrician, only with more emphesis on damage and less on defence. As a result, you should have little issue hitting top score as long as you're placing your trap in the right location and popping your focus shot as much as possible. Because of your reduced crowd control, it's a good idea to hang out with your teammates rather than being a one man army. The only way to improve this class is to rework the focus shot to be a little more fun to use.
The Survivalist is a hardcore rusher. Being able to go invincible and having one of the best defensive traps means you can really push yourself for most damage. If you don't apply yourself though you can easilly fall down in the kills, and it's easy to get complacent and camp at your trap. A good survivalist should never die but a great one should risk death every second. Its trap is maybe a little too strong and the sheild is far too easy to use. Change it so it only blocks/deals damage in the direction you're facing and it can remain as strong while being way more fun to use. You can use the sheild and a melee weapon to quickly take out the boss.
The Scientist is... well kind of a joke. It has by far the worst defence until the black hole is upgraded and the combo ability is pratically useless. However, once you to put your time in, the scientist can simply pop a black hole and sit inside of it doing nothing while taking out anything too close. The explosive potions should throw far quicker, deal less damage, be easier to aim, and the black hole should be reworked to have a much longer range (bigger than the turret) but deal no damage, instead bringing them all to the center where you can use your potions to take them out quickly. Alternativly it could act as very slight chip damage to all zombies in its range. If you want to challange yourself, try soloing the boss with scientist. I bet you won't be able to kill it before the time runs out.