This guide is about things that I've learned through playing the game. There're always different solutions and possibilities, the game can be probably finished on the hardest difficulty even when not following it.
It's also by no means finished, I'll be adding pictures so it's not a huge wall of text, as well as fixing typos and adding information as I come accross them.

The keyword for your party setup in higher difficulty should be versatility.
These are the basic aspects that can make your party closer to optimal:

• Damage
  
• Control
  
• Healing

Damage
It's not just the amount that counts here, but the variety of it as well. Encountering certain wards, or enemies that are simply immune to certain damage type can make an easier fight harder to win.
Early game you encounter a lot of skeleton and undead, which happen to have fairly high piercing and slashing resistance. That's worth to keep in mind during character creation, since the early few levels you don't have as many tools available to you.

Control
Control over your enemies and environment is just as important as damage, especially when you're doing a run with Lone Wolves.
There're many sources of control, such as magic schools, grenades, special arrows, barrels, environment and Man-at-arms skills.
Few of the most powerful ones are Charming, Freezing, Water/blood surfaces and smokes.

Healing
While it's nowhere near as important as the other two, it's good to have at least off-heals. The healing potions and food are fairly easy to make and action points-wise cheap to use even in combat. Generally you want to avoid taking damage, rather than just overheal it.

Smaller aspects would be buffs, crafting and movement - which you can also benefit from.

The most important tools that have been added to Enchanted Edition:

• Grenades
  
• Wands
  
• Dual wielding

Grenades
They're absolute beasts, but they do lack range.
I recommend using them with a melee character to affect as many enemies as possible. Grenades may seem underwhelming initially due to inaccuracy of them, but I found it's absolutely worth to get the talent "Pinpoint" as soon as possible, if you're going to rely on it (you do have to sacrifice 1 point to Scoundrel to it though, but I found them to be extremely helpful).
Pinpoint does what it says, once you get it, the grenades are never going to miss.

Nail bomb, Toxic and Firestorm grenades are very cheap to both buy and craft. I recommend looting and stealing empty bottles, ropes, fuses and buying as many grenade-related ingredients as possible from Aurelius in the barracks.

Here're a few of the more important recipes for grenades:
Small Poison Bottle = Empty Potion Bottle + Toxic Barrel
Toxic Grenade = Small Poison Bottle + Empty Grenade
Fuse = Rope + Rope
Bottle Filled With Oil = Oil Barrel + Empty Bottle
Firestorm Grenade = Bottle Filled With Oil + Fuse
Nail Bomb = Nine Inch Nails + Empty Grenade
Frost Grenade = Water Essence + Empty Grenade

I don't really recommend crafting Frost Grenade, as Essences are very expensive, somewhat limited in numbers and otherwise useful as well.
Throwing Toxic Grenade and Firestorm Grenade at the same spot obviously results in a bigger explosion.
Nail Bomb does very good damage as well as very strong bleeding damage. The opponents bleed a lot from it, which works as a conduit for any electric damage (as in possibly stunning a whole lot of enemies for turns).
Frost Grenade freezes a rather large area with mobs on it. The ground remains frozen for quite a while, making a bunch of enemies unable to move (extremely useful against melee opponents). Once the ice finally melts, the water also remains for a while and air damage on it will stun everyone standing in it. (You can even throw a Firestorm Grenade on top of that when it's about to end, there'll be steam, which also results in a stunning area when there's air damage done in there).

Wands
Powerful ranged single-handed weapons with certain elements. They are Int based, so their damage also increases with Int. They don't have a chance to miss like other ranged weapons.
There're limited use skills on them, which you can't replenish - but they're not really that important, although can be potentially useful early on.
Wands are excellent to interact with environtment. Fire Wands can blow up poison and oil, air can stun enemies in water, blood or steam.

Wands can be crafted just as any weapons:
Fire Wand = Inert [something] Wand + Fire Essence

Obviously different essences result in different wands. There're different Inert Wand types as well, I imagine they simply indicate the quality level of them.
Not sure if related or not yet, but crafting a wand with level 5 Crafting ability resulted in a 4% critical chance bonus on it.
There's probably also a way to boost the damage of wands through crafting, but haven't found it yet.

Dual Wielding
Rank 1,2 and 5 are reducing the amount of AP required to hit by 1,2 and 3 respectively.
By increasing ranks they're also reducing the damage penalty when dual wielding - the penalty is completely nullified by the time you get rank 4 Dual Wielding ability.

The damage boost stacks with the Wand ability, which makes dual wands extremely powerful. It allows you to use multiple elements which makes control way easier.

It does not work the same with dual wielding one handed melee weapons. They're still powerful, but I've yet to see situations where they're significantly stronger than having two-handed weapons. With dual wielding your attacks do require less AP, but the damage doesn't seem to be much higher and you sacrifice range (may not sound important, but two-handed weapons have better reach over dangerous environmental effects as well as Opportunity attacks, I imagine).

I'd like to share a few "tactics", which some may consider common sense, but might be able to help a few.

A list of our tools would be:

• Chokepoints & Line of sight
  
• Traps & Delaying turn
  
• Pyramids & Fleeing
  
• Minor tips

Chokepoints & Line of sight
You don't always have to rush into a battle. Sometimes it's worth to start a fight and then find a place where you can fight on your own terms.
These usally have to do with doorways, walls and anything that can break the line of sight of ranged characters. If no such thing is available, you can use environmental effects which result in smoke.
All that makes the enemies move a lot instead of attacking you, making it easier for you to deal with the rest. If you can set up harmful surfaces right in front of a door, the enemies will have to walk through it to reach you. By the time they get through, they'll most likely be damage and you'll be ready for them.

Traps & Delaying turn
Oil barrels, Toxic barrels, Oil flasks, Midnight Oil, Nail Bomb, Boulder Bash, Rain. Just a few of of many tools available to you to set up traps easily on chokepoints. Wait for the enemies to reach them to blow them up, stun them or harm them in any other way!
I guess it isn't utilized, but there's actually a skill for Delaying Turns (you can get it on your skillbar, when pressing 'K' for the skills, it's a shield icon among "Special" skills). This simply puts you to the end of initiative list, allowing you to do your turn after the enemies moved up - potentially right in your trap. Additionally also your heroes usually have higher initiative than most of their enemies - in which case you will get to start the turn right after the delay.

Pyramids & Fleeing
Apart for a great freedom of movement, pyramids have more uses. You can teleport through the battlefield for just 6 action points. Might not sound so amazing initially, but it does help you with setting up traps and then delayed turns.
Might sound a bit of a cheesy move, but you can actually flee off the battle with only one of your characters in their turns, as long as there's no enemy nearby. That character will be able to move freely, out of turns. You can basically heal up, replenish your stock of items and buff yourself - then use your pyramid and join the fight again.

Minor tips
Some enemies have animations which can result in you missing an attack due to misclicking. A better way to avoid that is checking your game settings so the enemies are highlighted - at least in tactical view.
Tactical view (default keybind should be 'b') will change the camera close to a top down view, which makes you less prone to enemies wiggling around out of their turn, also makes it easier to see the extent of certain environmental effects.
You can also get used to attacking with Ctrl+Click, which is basically ground target, but I feel like it highlights the target better.
In case you still manage to misclick, right click can interrupt your movement, so your action points aren't going to be wasted entirely.

• The importance of crafting
  
• Dual Wielding versus Two-Handed weapons
  
• Traits and immunities

The importance of crafting
Earlier I've mentioned how powerful Grenades and Special arrows can be, but that's not all you can do with crafting. You can also improve on your gear - as seen on the right.

That is a pretty solid two-handed sword for its level to begin with. But through blacksmith you can easily add quite bit to its base damage, with crafting I've added bonus air damage, strength and dexterity to it. These effects are also greater with higher level items.
But that's not all you can do, you can also improve your armor and resistances as well. With Nine Inch Nails attached to your boots you can be permanently immune to slipping (which means a lot in Hiberheim or whenever you throw a Freezing Grenade).
There's a bunch of other things you can make, early on for example it's useful to craft gear with stat boost.

If you're going to craft, then get used to picking up everything and buying all the cheaper materials from the traders. You don't have to carry them with you, I usually just drop off the heavier stuff at the Cyseal crafting stations (it's worth to keep a barrel of each kind there as well!).
Crafting can also be used for making money - just create some basic items and then sell them. You can basically make items worth 200-500 gold for materials you get for 20 gold. Now that the traders refresh their stock every hour, you should never really run out of gold.

There're items with crafting and blacksmith bonuses as well. If you get those early on, and you have Pragmatic trait, then you'll only require 2 points in Crafting and 3 points in Blacksmithing.



Dual Wielding versus Two-Handed weapons
In case of Man-At-Arms character my personal experience is that Two-Handed weapons perform better.

Yes, Dual Wielding costs less AP and they offer a better variety in terms of effects and damage. However it does require you to find multiple one-handed weapons that are pretty strong, whereas with Two-Handed you only have to be lucky once.
Furthermore two-handed weapons have better range. It may not seem much, but it means extra range on attack of opportunity and the possbility from attacking without entering environmental effects more often than not.
Dual-Wielding also requires 5 points early on to achieve the desired AP reduction, whereas the critical chance and critical damage boost isn't as vital early on - but it does help a lot nonetheless.

And finally the Man-At-Arms skills seem to be scaling better with two-handed weapons, from my experience. They require the same amount of action points and have the same amount of cooldown times - except that from Two-handed weapon mastery you get quite a massive damage bonus on critical hits.

Traits and immunities
This isn't suggested for anyone that's actually role-playing in this game.
It's fairly easy to predict when you're about to receive a trait point. The dialogue will offer two options usually which are pretty much the opposite of eachother. If you know all the traits, it's somewhat easy to predict the outcome of each option, but if you want to min/max then you can just "cheat the system" by saving before picking an option - if you're unhappy with the results, you can just reload the last save.

For some reason these traits are incredibly powerful, which isn't really all that great for role players in my opinion, but well...
Through picking the right traits, you can get immunity to Charmed, Fear and Cursed - which are among the most annoying and powerful crowd-control skills.
There're a couple more worth mentioning, such as the +1 Willpower, Leadership, Crafting and the +3% Critical chance, but they're nowhere near as noticeable.

You can also get immunity to slipping on ice through crafting. There're bunch of items which offer other immunities (on level 10, aside from those 4 immunities, I was also immune to being frozen and crippling as well). These can be game-changer effects.

Useful links
I didn't want to make the guide too repetitive, so I'll just try and fill the missing information through linking other sources of information.

Divinity Wikia[divinity.wikia.com] - A standard wiki site with a bunch of information. You can still use the original DOS part of the wiki as well, as some things remain the same (Homestead, many quests, locations and so on)
This very handsome looking pdf file with the original DOS recipes. [www.divinityoriginalsin.com] Doesn't have the EE recipes yet, but it's very detailed with all the icons. Can't really tell who the owner of it is, as I've came accross it through googling for a recipe.


Guide changelog
04/11/2015 - Added "Useful links", "The importance of crafting", "Dual Wielding versus Two-Handed weapons" and "Traits and immunities" articles.