I decided to just focus on the three roles of:

• Tank
  
• DPS
  
• Healer

Obviously there's a lot of other ways of playing the classes but since each class usually have 3-4 different ways of playing their roles with crossover-roles and multi-classing this is just the fundamentals to get you started with how to think about them. The fun part comes from experimenting, for this I advice writing/reading on the forums there are a lot of fun discussions there about strategies.

This document is very bare bones for beginners.

Brawler
Pick 1-3
His passive makes him Tanky giving him a shield with 9% of his max hp half of the time but also ok DPS early on. Focuses a lot on returning damage. I don't like or dislike him, hes ok. I tried to build him to return as much damage as possible/damage with his active but its hard to balance that and also make him tanky.


Gladiator
Pick 4/5
Gets free HP with his passive which usually gives you around +700HP at the end which is awesome. Lackluster in damage though, basically "just" built around tanking and early on your passive will give you like +50 HP on the first map and you will be left with a pretty useless early unit. Can be saved and picked up as unit 4/5 and given two or three Giants Club which does damage based on your active HP for 150*3 damage per hit if you find the items for it.


Juggernaut
Pick 1-3
Very robust. Very nice passive making him very tanky and giving him a bit of DPS. The hardest part with a tank initially can be that they just absorb damage. Juggernaut will give you a lot of early wins due to his ability of dishing out ok damage and still tank. His passive is good into late game too with the +60 DEF.

Also has an active with a stun, which is great. It also only increases in damage when you upgrade it so you don't need to spend 200golds worth of ability orbs on him to have that aspect maxed out as well.


Fiend
Pick 1-3
A bit of a memepick for me. Does not lose morale when knocked out and comes back after 7 seconds. He works ok but has horrible HP/DEF for prolonged tanking. I pick him up if I have a tank that I want to charge up for 10 seconds before he is supposed to go online. Fiend takes the first couple of hits for free and gets knocked out without me losing morale and hopefully he give my tank a couple of seconds extra to get his shields or max HP up
People in the forums have made him work in a lot of different way but he's not really my cup of tea.


Ranger
Pick 1-3
I don't really play the ranger that much but apparently he is S-tier according to a lot of people. Gets a pet with him that either does DPS or tanks and most people use it as tank. The pet gets 150% of the rangers DEF stat and you can easily get 3-400 DEF on it very early on but I feel that then the ranger just have to stand in the background with a bunch of DEF-items making him good but super boring.


Paladin
Pick 1-3
I go paladin if I go shield or fire strat. He is an OK tank, not awesome by any means but OK. The reason you should be getting him is his passive though that when fully upgraded does 120% fire damage per hit on all adjacent enemies. He is the only unit with aoe as an attack and its fun to play around with. He also has a nice active ability giving all of your guys a shield so even after he gets knocked out your team will usually have max shields to tank as well before they go down.


Illusionist
Pick 1-4
Could also be on the DPS-section but I usually see him as a tank with the enemies fighting the illusions.
Usually requires heavy investing resource wise. Is sadly also dependent on luck. His passive is supposed to make him teleport away and skip taking damage 30% of the time but in reality it feels more like 10% and there are a lot of fights where he will just die without teleporting and achieving nothing.

If you decide to play him then get items that carry over to his illusions, this is HP, ATK and DEF. Skip everything else except the symbiote item and he will spam images with 1100HP once every second. Even on awesome runs this is very very luck dependent though.

Pyromancer
Pick 1-3
Insane spike damage. Insane! His active takes all of the burn effects, doubles them and attacks with piercing damage, bypassing all defense which usually removes around 15-25%. Double damage from all fire basically. Can be weak early on since his active only does 1x fire damage as piercing but balls out of control after that.
Also good because he does not need any cooldown removing items as there is little point to spamming his active since not enough fire dots would have built up, freeing a slot or two for MORE fire!


Ice mage
Pick 1-3
A lot of people have this guy in tanky compositions with a frozen shell, giving +DEF the more frotst is in play. If you get him that will be a core item that will provice you with +70-300 DEF on your main tank. Also nice with an AOE-stun. Pretty poor in damage but he will provide a lot of space for your damage dealers.


Wild mage
1-3
Super fun until you run into an enemy with retribution (return X damage per hit). Has a passive that decreases mana per cast which is awesome. Very quickly becomes a minigun thanks to this if you pair him up with other cooldown removing items.
If you get wild mage also go cleric or see him blow up within 3 seconds from retribution.


Ronin
1-5
Very nice passive but with small proc chance initially, upgrade this at least once asap. Even better active that does damage to all enemies adjacent to each other and increases in damage per cast. Again, focus on item removing cooldown and damage. Alternatively give him an attack speed item and watch him put the enemies in a permanent disarm. Doest not work on bosses though, but his bleed does.


Berserker
1-3
Very good DPS, free damage scaling AND life steal for free. Pretty lackluster active but it activates his passive meaning he will build up attack even faster.

Some people might think he will be a godsent tank but I have not made him work as one so far on the later difficulties. He likes to chop people and not to be focused, help him do so. Get him attack speed items ASAP the rest is up to you just make it scale with his damage.


Assasin
4-5
I always get this guy last.
I would LOVE to get him earlier because he is awesome for DPS but I usually always need to fill the last "gap" in my strategy with hero number 4.
Assasin is stealthed which means he will never get attacked unless it's by AOE or other all heroes are dead meaning you can give him ONLY DPS items and put him wherever you want.
His passive is amazing, it might not look like much but poison never runs out and if you get a lot of stacks up quickly the enemies will be dead as soon as they hit half HP if your heroes are still alive.

Obviously he is not as good vs a lot of smaller units but he is still OK.
Has a good active as well. Even better is that you do not need to upgrade it as it just increases


Alchemist
1-3
Really nice active, when you cast it on max it will take 9 random poisons and double them. If you have this with 10MP/cast you can say he creates ten new poisons per second!
Like the rogue one of those heroes that will do 70% of your total DPS but you wont see it.

Since it's random what poisons he doubles I would recommend items that gives those rare 50damage per tick and to NOT give him any items with plus attack speed and skip out on the assasin. His passive will do 5 damage per poison tick and the assasin will do 10 damage per tick. The less of these are in circulation the bigger the odds are that he will increase the poison stacks with 9 times 50 poison damage every two seconds...every second. This is where the numbers just get nuts, you will want to try this at some point when you get the run allowing it.

But even without doing this he is still incredibly powerful and a core pick for damage dealers.


Outlaw
4-5
A half-rogue/tank kind of.
Will take a couple of hits and THEN stealth away to safety. This can missfire though if you get unlucky with heals and the enemies get some poisons in or whatever and actually kill him before his stealth can proc.
Apparently very good but I haven't played him that much I kind of feel like other people does what he does better.

Druid
Pick: 1-4
Has a powerful heal over time that also gives damage and negate.
The drawbacks are that negate does not remove any debuffs already applied and the heal over time will require quite a few stacks to be able to keep your guys alive. It's sadly pretty common in my plays to see him spend 10 seconds to cast heal 2-3 times on a hero that is obviously seconds from dying instead of preparing the next hero to get pummeled. if you manage to get it working it's really good though. Almost always in my team.

Also a nice passive allowing him to tank a little bit early on if you really need to, not worth investing in though.


Cleric
Ok so this guy is quite special even though he looks a bit boring. I kind of feel like he should be called Destroyer of worlds in some of my games.
He has a pretty straight forward heal, 30% of max hp which when upgraded is very good. Usually you need to skip this go get your mages damage online first though.

So he also has a passive that does 50% damage to random enemies when a heal is cast. Big whoop, right? That's like 20 damage every 5 seconds. And there is already an item doing this that you can get.
Well, you can upgrade this two times to 150% of ATK per 5 seconds. Still not big whoop? Ok ok, well you will obviously want him to hold on to an item that allows him to spam the heal once every second, right? That's 150% attack damage per second for each heal but thats not even the most broken part..
Read the description again, this is per any heal that he does regardless of source.

Time for numbers!
The item Brain sage heals a random hero for 100HP every 50MP you spend. It does so in 5 "ticks" of 20hp each. That triggers the clerics passive 5 times! That's the damage of 5 heals every 5 seconds just holding on to this item. And there's more. There is an item called Life Giving Chamber that in itself is broken. It damages the holder for 50% of their HP and then gives 75% of the taken HP and gives it back as a max HP buff to the other heroes. It also gives them a heal over time each. There is also an item called maidens Mirror that takes any source of healing and gives 90% of it to another hero.
Meaning that if you have this item you will START combat with 8x150% ATK every 2 seconds for 10 seconds.

Cleric is the potentially most broken hero in the game. He makes sure you survive and he makes sure they die. There is no reason not to have him in the party except for challenging yourself.

Just pay attention to the damage meters, you will have your Gladiator in the front holding the head of your slain foes mocking them and getting all the limelight but the numbers don't lie the cleric in the back did 70% of that damage by healing. Cleric doesn't need that fancy stuff, hes here to do a good job and that's what he does.


Bard
4-5
The antithesis to the cleric. Crazy good but also feels like you don't get another hero you just get a healing ward. He can either heal or give your heroes damage which is amazing but he does nothing else.


Bloodmage
1-3?
I have not played this character too much which made me a bit unsure where to place him. Apparently S-tier again but I just cant get around to liking him.


Warlock
1-4
Ok healer, meh damage dealer. Have not played that much, he was my secondary healer initially.
The only healer that does a fixed amount of heal instead of percentage which can be good if you have a tank with a high DEF and low HP (this almost never happens though).
His passive can make it sound like he is supposed to tilt the scales to your favor when all of your heroes are down but I would say that all it does even when fully upgraded is to shift a combat where he is the only one left with a monster a tiny bit. Basically it means that maybe he can survive 2-3 seconds longer, usually which is sad because I kind of like the idea of him being super strong when he is solo.