DISCLAIMER: Just remembering, I’m posting here because it’s the place to discuss “game industry”, it’s not a defamatory post about some company or game in special, but I think here is a nice place to discuss (and laugh from) some game design choices. 
[FONT=Book Antiqua]If you don’t know the term, Artificial Difficulty is the direct result from us bending and using our own game rules to make something look and feel harder (and boriest) than should be to our players, well, it’s easiest than plan and balance an interesting combat, so if we get lazy to think into something original and good, we can save a lot of time using these simple tricks!
1 - The two rules’ Rule:
"Our bosses and mobs should obbey a diferent rule set from the one applied to players."
Keep in mind that It’s not about JUST a simple increase of twice the life points, three times damage reduction, and five times damage dealing (that’s newbies trick and could be justified as game balancing process), we really need to create someting that JUST AI enemies can have and use.
As example, make the player “think” he is playing CHESS, so submit the player to the “known” rules…
Now, let’s allow our AI to use ALL pieces like being Queens!
We can also add some “extra” challenge to this, maybe the AI pieces just could be beaten while they’re red! A more “actual” example should be constrain the player to just attack “efectivelly” at close range, but give long ranges options to most bosses.
Well, we can argue that this goes against all the concept from AI Opponents right? But let’s remember that we are not here to design games where the rules could be stablished and make sense. We are here to design a “chalenging modern” game.
2 - The brain handicap’s Rule:
"Make a good AI to give brains to our games are hard so, let’s constrain the player brain to just what our poor AI can manage."
Of course marketing a game with the word “Big Environments” is a nice thing, but since our marketing departments don’t help us to design AIs capable of find a player on a huge scenario and send our monsters/mobs/bots there, it’s easiest just “frame” the player inside a clear and plain room each time we want put him to fight.
About bosses, well, our artists expended a lot of time modeling that “HUGE” (did I said that lo look challenging bosses needs to be huge?) monster/robot, so it’s better this time worth! Every time we decide that a battle needs be “Epic” the player MUST BE LOCKED. If our AI can’t find the player, let’s make the player just be allowed to move where it could be easily found and reach.
So, no matter if the player could break or slice doors, on these situations, a “MAGIC” barrier will limit him to something that our AI could manage.
3 - The AoE’s Rule:
"In the doubt, AoE."
Note: Don’t mess this rule giving any AoE to the player!
Did I said that it’s too much hard to our AI programmers make something that could hit a player? Specially if we gave some sort of dodge move to it, ouch, our game will be flagged as too much easy! How to correct this?
The clear answer: Area of Effect skill/blow/ability! No challenging mob from today (specially on consoles) could be called “challenging” without a hit that could not be evaded. This is a nice opportunity to increase the time expent on our games let’s force the value/hour thing!
We can use an animation to tell the player that an AoE is coming, and so, the player will need going away (to the max he can go inside a room, remember the Rule #2), wait or get smashed! It’s brilliant! And so we can call this warning as feedback!
Ow, this could be used to the most daring evade until the most deadly things… We need to solve this!
4 - The be sure it will hit’s Rule:
"We said HIT!"
Well, our player is already constrained to something our pathfind system can manage, so why don’t use this pathfind also on skills/abilities/blows?
Remember that if the player evades our AI attacks, he will beat our game too much shortly! And so to correct this we’ll need REALLY to put content on it to keep him entertained… What an insult!
So, let’s use our poor pathfinding at our favour, we just need put our projectiles to look for the player while the ones shoot by him (following the Two Rule’s Rule) need be targeted, and maybe loaded before shoot!
Also i’ts easy to our programmers to rotate an AI unit, so, let’s make our bosses also keep rotating and face the player’s avatar “while attacking”! This will grant a hit whatever that small plague do!
5 - The as Jesus Christ’s Rule:
"If all fails, let’s make it ressurect on the third day."
Well, let’s remember that we are trying to keep the player “entertained” by the bigger time possible, whatever if makes sense or not, so what about put some kind of regeneration skill/ability/spell on bosses?
As everyone knows one of the greatest sources of enjoyment to any human is to watch whatever their efforts produced going back to zero! So we can trigger some regeneration skill if all our previous “cares” didn’t sort the desired effect.
But remember that we are not talking here about some sort of “seccond form” or “evolution” from the Boss (this will make the player feel he progressed, and so they will not say our game is hard) we need to make it in some way causing the player feel a loser and all his efforts were useless.
6 - The swarm of trash’s Rule:
"If our enemies just worths a cent, a hundred of then will be one dolar."
If we are following these rules a new and great horizon will be open to us! If we follow the tip about don’t give any kind of AoE to our players, they’ll probably enjoy be corned by a small army (more than six please) of weak enemies. They’ll probably be happy on die to then.
This is the most “creative and original” way used by “us” modern game designers to make until the most ordinary part from our levels challenging!
We can increase this technique efectiveness getting these enemies on combined groups (range and melees together), so while the player deal with the short ranges, the longer ones get easy hits.
7 - The automatic and granted retaliation’s Rule:
"If I go, I’ll take you with me."
That’s one of my favourites! It’s undeniable that soon or later the player will hit my precious AI enemy, he must be punished by this!
Well, again it’s a nice way to save some effort on AI, since the player will for sure find, target and hit his opponent, it’s a nice idea to save the effort from our AIs making the inverse way and simply using the player effort against him. So, his “hit” can also make the work from our AIs and damage him in exchange.
Well, the “thorns” effect as some people got used to call this an have some variants, maybe we can apply some “cosmetics” above it, maybe the player could be “parried” some times and getting “stunned” time enough to get a blow from our marvelous AI.
A small artistic touch to the parry thing is to combine it with Rule #1, the player parry (if allowed) can’t stun our AI!
8 - The fault is from camera’s Rule:
"Who said that what eyes can’t see, heart will not feel?"
Let’s remember that we are on the 3D era! So why don’t use this also against the player?
Different from our AIs, the player must “REALLY SEE” what he’s fighting, so if we can get him lost on the camera control, better to our difficulty right?
It’s a thin line between use the camera this way and make the “game unplayable” getting this pointed as “bug”, so use with care. There are lots of possibilities to this if used right…
As example you can constrain most of your gameplay to enemies on ground, BUT add some flying units on such angle condition where the player will be getting damage and fighting either ground or air units (remember Rule #6).
Also the camera “focus” is a great thing (while that thing of Rift doesn’t become widely used), you can place your long range units out of the screen, so your players will have most of time damage coming from somewhere.
9 - The Big Brother AI’s Rule:
"We know everything!"
- Suggested by @inside
Make enemies behave like extremely intelligent and agile players, to infuriate your players ;>
On top of the fact that they have access to everything player have (weapons, abilities, items), they also have access to data which is hidden from player (like exact information on action performed by player, exact hp values, attributes etc).
On top of that make AI totally unpredictable. So it won’t ever use the same tactics twice, or use best tactics possible every time. Make it as random as possible.
Also if your game has some sort of elemental attribute protection, equip your unit with all types of spells/abilities/skills, so make an “under the hood” check to get how to hit your player more effectively.
Following these rules will make from our games a great success by what I’ve seen, otherwise, I would not have been played too much games using such things on them.