Introduction! I've seriously tried writing the intro to this eight times and hate every version, so let's jump right in. Balancing encounters (for challenge and fun) really falls into three parts: Planning, on-the-fly, when the dice misbehave.
Planning Encounters:
1. Don't use one big monster. It never works well. Single monsters go down too quickly or not at all. In the latter, you're asking for a party wipe. Bosses can have minions to run interference, just give the party more than one target to give them something to do.
2. The books have a fair number of tools for planning balanced encounters. Use them.
3. Try to have enemies that mirror the party's abilities. The rogue wants to flank and sneak attack. The warriors want to crush skulls in melee and fell foes en masse. The ranger wants to take out key targets with precise attacks from afar. Clerics heal, mages incinerate things.
4. Error on the side of too many too weak enemies rather than too few stronger enemies. It feels more heroic to charge in and cut down five goblins rather than spend the entire battle dealing blows to a single ogre.
5. Plan varied encounters. Something tense to glue everyone to the edge of their seats followed by a much easier encounter to build confidence is rather effective. Look up the Star Wars tension graph for how you should plan encounter challenge levels. (Moderate, easy, moderate, slightly challenging, easy, slightly challenging, tough, moderate, and so on).
6. Keep notes to avoid cross-referencing to a minimum. You're driving the game, try to keep from pulling over to check a monster's AC or attack bonus.
7. Plan for variations. This ties into the next part - you want to be prepared with a slightly different encounter if things start going poorly for the party.
On-the-fly counterbalancing:
1. If a monster goes down too quickly, give it an extra 50% to its hit points to make the fight last longer. If the party is struggling, go the other way. The players don't know the monster's HP, so this is one of the best tricks. If the party knows this type of monster's HP (roughly) then give an in game reason for the difference (e.g. it has deep cuts from a previous battle).
2. When setting up a battle, decide how many enemies to send at the party then. This shoild vary from your original plan to balance for the party's current state. If the last battle was tougher than expected, take an enemy or two out of the picture to give the players a breather. Again, this works great because the party doesn't know what you planned, so they won't notice the difference.
3. Battle is too easy? Additional enemies are easy to write in (they heard the battle and came to help their friends).
4. Battle too hard? If you fudge numbers, don't give any indication that you did so. Better yet, don't fudge numbers (maybe ignore a poorly timed critical hit). If you suddenly start rolling poorly every time the party is in dire straits, they'll know. Players don't like this. It removes all tension from encounters. Let bad things happen - just don't go for the kill out of spite. Remember, they can always run away... if they're not trapped.
Bad dice:
1. When a player consistently rolls crap, give them another roll every once in a while. "Your deity smalls on you, granting you another chance. Roll again." Or maybe, "You strike hard against the bugbear's shield, throwing off his guard. You may take an attack of opportunity." This seems to contradict my advice, on dice fudgery, but players appreciate when you throw them a bone from crap dice. The dice can ruin their plans - and their fun. Just don't do it all the time.
2. When you consistently roll crap, suck it up. You're not the hero here. Unless it's a battle between NPCs that should be awesome at what they do. In that case, why the hell are you wasting the players' time by rolling?
3. If you consistently roll well, let the players hurt. If it will be a wipe, start plotting how they survive that doesn't involve fudging the dice. They were captured, rescued by NPC, saved by divine intervention, whatever. If all else fails, give them a one-time do-over of the battle (divine intervention?). My point is there are better ways to avoid a party wipe than removing all sense of danger by clearly fudging rolls.
See you next week.