The proposed system for building combat encounters (as found in the online DMG basic supplement) results in encounters that are too easy. It is possible that it is appopriate for first level characters which are disproportionalty fragile in this edition compared to the lsat, but it breaks down very fast as the party levels up.
Some observations :
1) A party can handle as many monsters as it has members, plus one or two more. Encounter difficulty decreases slowly as monsters are added, and increases dramatically after the party is outnumbered two to one. Encounters become dramatically easier when the party out numbers the monsters.
2) A first level party is very fragile, and probably needs its own encounter building guidelines. A second level party is still coming into its own, but by third level characters are no longer depedant on luck to surive a tough encounter.
What this means for the guidelines :
1) The mulplicitave factor for monster size is way to high.
2) Smaller groups of monsters allow for much tougher monsters.
3) You really have to add a lot of smaller critters to get a hard encounter
4) There probably needs to be an adjustment for monsters above or below the parties tier; as the monsters proficency bonus affects hitting, getting hit, and making saves.
Keep in mind that the guidelines are (potentially) still a work in progress since the DMG has not be finalized and sent to print yet.
Also keep in mind that you may have a skewed viewpoint because your group of players may not represent the baseline group to which the encounter guidelines are designed, since player choice and character build have a dramatic effect upon the difficulty which encounters appear to be in hindsight.
That said, I find that parties of all levels that I have run for in 5th edition survive primarily on their intelligence, and when they mis-step a back-up plan of lots of luck - every encounter I've run so far that wasn't dramatically stacked in the party's favor has come down to the players feeling they were in serious danger, had their plan or the dice not worked out.
AaronOfBarbaria wrote:
Moot_Dundle wrote:
I have noticed that tactics an surprise play a huge part in encoutner difficulty. The same monsters that are easy when encountered in a room where the squisheir PCs can stand behind the doorway will become extremely difficult if they can lure the PCs into a trap, splitting up the party formation and circle around behind. If your players are smart, or your encounters are laid out in a way that makes it easy for them to take advantage, the standard difficulties may be too low.
JasonDenton wrote:
I think a good skill to have is to be able to build situations that are challenging by the numbers or by ignoring the numbers. If hit point whittling is the solution the DM is pushing, then it tends to be easy as characters are often built to be good at hit point whittling (and mitigating their own hit points being whittled).
What if the enemies are all 1-hp monsters that do no damage but can still win? Seahorse Run.
What if the primary antagonists have better things to do than attack adventurers - and can still win? The Corrosion of Gedren's Point.
What if the players' strategy escalated the encounter? Four-and-Twenty Black Things Baked in a Pie.
Guidelines are guidelines, and depend on your players and their characters.
That "easy" group of Intellect devourers can absolutely devastate a Fighter, Druid, Ranger, Sorcerer mix, regardless of level.
That "Hard" group of Cold Vulnerable elementals are easily wasted by a Cold Dragon Sorcerer and his Evoker buddy, especially if they know they're just gonna go to sleep after this battle (and not have to "hold back" due to spell resources).
Those "Challenging" Hobgoblins are nice and easy if you have a group that, collectively, has superior tactics than you the DM (which happens a lot. I'm only ONE person!) and/or gain surprise
That "Deadly" Mob of kobolds are easily wasted because you accidentally grouped them together for the perfect Fireball/Evard's Black Tentacles/Spike Growth
So! Again, this works great as a guideline, but it's pretty hard to be all-encompassing when every party (both the PCs AND the players) is different. Also, it can be about resource management, too. We don't like having combat-focused campaigns, so when they get into fights, they are often more difficult since they're not dungeon crawling. If they're planning on taking a Long Rest after this, I crank up the difficulty if they have lots of resources left. Or maybe interrupt said rest as a way of saying "Hey, don't meta-game so much, k?"
But hey, that's what the DM is for! Adjusting where flat rules/guidelines cannot. We're people, not computers 
(also, as a note, I hear from most people on these boards that more monsters is much harder, almost disregarding HP if you play it right. Control works well, but so much "wasted damage" adds up. Especially if said mob either uses superior tactics, the help action, or have nasty effects! A mob of lower CR dudes is harder than a single, party-equal CR monster, from how things seem to shape up, due to focus fire)
Theros wrote:
I agree with Goober in that I am finding encounter balance to be very situational (and I like that). For example, my five strong party of 2nd level heroes seemed to be making mincemeat of the Redbrand hideout in LMOP - slaughtering the supposedly tough bugbears and laughing at the drunken party of four redbrand guards and the crypt skeletons. But they had the drop on them. Suddenly they get ambushed by a couple of redbrands and now they are down two fighters -unconscious for three hours - stuck in the cell room, desperate for a short rest but with three guards led by Glassstaff now hunting for them. It's going to be tough for them to escape the slave ships I reckon!
I agree with most of what has been posted, but remember for the most part, the creatures have an environmental advantage. The owlbears know the woods, and can use them to ambush the party. The drow do not need light in the dungeon, and can see the party with their torches or light spell easily; allowing them to hide and surprise. Use surprise rounds if your players are not having difficulty with encounters.
Another thing you can add to an encounter is traps. The party squares off against the Redbrands, but they're to busy fighting and don't see that some of the tiles trigger kissing maidens. Now, Ragner charges the ruffian across the trap covered floor, and must make a Dex. saving throw, or take the damage and be pushed back; preventing him from reaching his target. Adding traps and diseases can make a lower CR encounter intense. When the party faces the undead, they don't know that when the zombies are at half health(bloodied in 4th edition, which I'm still a fan of using to affect encounters), a crippling disease leaks from their wounds and 'poisons' party members within 5 feet that do not make the Con. saving throw. I know that Lost Mine of Phandelver did not have these traps or effects in it, but with accomplished players I had to up the difficulty of the adventure(plus it's more fun for me as DM; not all straight up fighting).
So much of a combat encounter depends on the die rolls, the tactics used and the individual PCs that all you are ever going to get is a very rough estimate at best. What is an easy fight for one group might wind up being a TPK for another.
The point is, if you are just relying on encounter design to balance combat it will almost never work out to what you envision, one way or the other. Sometimes it's OK to let an encounter be too easy or too hard, but if you want any kind of accuracy - designing combat encounters is only half the battle. It is critical that GMs learn how to adjust the challenge of a combat on the fly during the game session if they want to have any kind of in-game accuracy of the intended threat.
For example, if the party is just rolling through a fight that you feel needed to be more challenging, then there are a ton of ways to make adjustments mid-combat - increase HP or AC, have some more monsters join the fight, improve their tactics (like using focus-fire, ganging up or going for the soft targets), start using the terrain to their advantage, or even turn one of them into a caster and have him start slinging spells or something. You can also, of course, make combats easier on the fly as well through a variety of creative methods - many of which are the reverse of those listed above, not to mention good old fashion running away or surrendering.
Master those two GM abilities and your combat encounters will always be the "correct" difficulty.
JasonDenton wrote:
I'm sure most of you are already aware of the AngryDM, but if not I highly recommend the recently completed series on combat design (as well as his other articles) on his blog - http://angrydm.com/
really shows much more there is to encounter design beyond monster XP budgets.
I think it really depends on whether you are using feats and multiclassing and how many magic items the party has. Player skill and party composition matter of course too.