Right, so most of these are probably known, but I felt compelled to post my own thread (since I have posted nothing thus far). I have some trouble in combat on hard (sometimes

). PS: I am in NO-WAY a pro or anything - just my 2 cents on this game.
1.) Oil Barrels - I convinced dudes to be my buddies, grabbed all the oil barrels, stacked them around the enemies that are far higher level than me and then proceeded to fireball the barrels. Enemies a couple levels above you won't be a problem anymore!
CAUTION: The fire on the ground seems to spread very far with a lot barrels. So yeah, don't expect a tiny pool of fire.
This is especially useful in the Lucullus mines. I was super under leveled there and had to use these to stand a chance.
2.) Co-op Sneak Attack - My friend and I control 2 characters each. So, we cast haste on ourselves with our mages, we go into sneakmode and summon some minions to give us a free headstart in the battle. We usually count 3, 2, 1, GO! and then cast on GO.
3.) Lava Co-op Teleport - We basically sneak into combat and then teleport/featherspell the enemies straight into lava (if there are some nearby) and instantly take out almost any enemy before the combat even starts. I therefore recommend teaching level 1 aerothurge to all your characters for a free teleport before combat.
4.) Keep Fire Resist Gear (+weather the storm for man at arms dudes) - If you have atleast 120% fire resist (-20% from burning), you can run into lava without taking damage. This allows you to collect gear there, or even fight in lava, giving melee enemies a REALLY hard time.
5.) tenebrium weapons - once you get the ability, never stack your weapon ability above tenebrium or vice versa eg. 4 points in handed and 1 in tenebrium is a waste. Try to balance those two skill points, since they act multiplicative on your weapon damage
Example with tenebrium weapon: 100% damage standard x 1.4 (4 points in single handed) x 1.4 (4 points in tenebrium)= 100% x 1.96 = 196% weapon damage. That is double damage.
NOTE: Eidolon has mentioned that it bow ability points might not boost tenebrium bows. I wouldn't mind if someone would test this to confirm.
6.) Archers - these 3 talents are devastating to almost all enemies, but require some caution in playing: glass cannon (double ap recover, half life), the -1AP attack talent (I think marksman lvl 5 thing) and stench (to give you some space, since its hard to survive -50% life). You'll have to stack up your constitution to improve the effectiveness of glass cannon since you want to increase your max AP, and a little more life won't hurt (it hurts without it!)
7.) Lucullus witch cabin - before leaving this quests for later, kill the mushrooms around it. This will remove the barrier and you don't have to go find the spell.
I'm mentioning this since the mobs out in the map are quite high level and you'll want the xp from the low level mobs in there before venturing further.
8.) Some quest breakers you should avoid:
A: Don't kill the guards outside of the lighthouse for the chest. They are required to finish the lighthouse quest in Cyseal.
B: Don't destroy the chest in the "companion" room upstairs the inn in Silverglen. You'll lose all your gear later with a quest
Non-spoiler summary: Don't go willy-nilly and destroy every chest you see if you can't unlock it. You might just break a quest or 2.
9.) Summons9.1) Elemental de-summoning - I'm not sure what the requirements of this is, but it is possible to reduce your cooldown of elemental summons to 1 turn while having enough AP to summon one each turn. When they are de-summoned, they release an elemental effect on their surrounding ground that is similar to their element.
So, I typically summon an air elemental before battle (to keep a turn ahead), position him in a nice tactical position during combat and then summon the more powerful fire elemental in a defensive position so that the air ele. desummons. This typically stuns a lot of enemies, providing a very cheap group CC as well as a fire elemental ready to attack in the first turn.
9.2) Hiding behind your summons - The geomancy spider can take a lot of damage. It is thus very useful to send into a group dangerous enemies to keep them busy while you take care of the rest. The elemental summons also do a bit of tanking (although not that much).
10.) Narrow pathway positions - If you ever find a narrow pathway, try to position your entire party behind it with your tanky characters just behind it and your summons inside.
Also, crowd controlling enemies in such a pathway is very easy since they all crowd at the area and are grouped up. I typically use that piercing ice attack with my mage, which typically freezes a mob or two and leaves a slippery slab of ice behind.
11.) Knockdowns - This is one of my favourite effects in the game, whether it is from knockdown spells or slipping on ice. It provides a 100% chance to hit as long as you are in range, so that all your characters can switch to their damage stances and still hit (only do so if it is AP effective)
12.) Charming - Assuming you have an archer (which is a damn fine magic user with the proper arrows), the charming marksman skills are amazing.
Not only do they allow the enemies to tank themselves and attack each other, but spell casters tend to buff your characters.
NOTE: Most bosses cannot be charmed.
So, there's my noobie trick guide. Hopefully someone finds something new here (experienced players won't) that'll make the game on hard a little bit more manageable (for noobs like me).