Howdy All,
I'm about to start running the new Elemental Evil campaign, and was wondering if I should add some creatures to the encounters. The party I DM for is about 6-8 players depending on the week, and they have absolutely crushed all of the other adventures WotC have published. In LMoP and ToD(HotDQ and RoT), I have barely been able to threaten them in combat once they reach level 3 or 4, though they say that the fights have gotten harder. They are very experienced group, expectually together(they've been a team 1-4 times a week for two years), and I really want to put them through the wringer. If anyone has ran this book that has any advise for me, I'm all ears.
Thank you
Giving all foes max HP for their dice is a good idea. I think I'll start doing that for my group too.
Thanks for the help. I increased the lower level fights by one enemy for every player over 4, and gave half of the enemies max HP. When they went after the Reaver Ambush fight(one of the players choose that hook from the book) I did not increase the numbers from the fight, and 3 characters almost died, as well as one of them was killed rather quickly(they were level 1 for the fight). Overall, the fights were intense and challenging for the group.
I'm a believer in using flexible HP for monsters, mostly because if the fight feels long, or it feels like every opponent is too similar, I would rather see the fighting end, especially if I can feel the direction it is going to conclude anyway. Try writing down HPs for your creatures with a minimum/average/maximum HP value, and then feel free to stray to either side of the average as appropriate if you feel it needs to be more or less challenging. Just be careful, as it is really just an extension of combat time if not used wisely.
If fights are too easy, then perhaps your monsters aren't hitting often enough, or hard enough. What can really scare them is when they start going unconscious (but especially their main healer if they have one). I find the players really pay attention when they feel like the next hit could take them down. That's when they know they need to, "get it done," or start making plans to retreat. That feeling of, "every action counts," is what I strive for.
Find each PCs weakness, and be sure to strike at it every once in a while. A front line fighter that gets suddenly sneak attacked with a critical hit...that's the sort of thing that makes it dangerous to be at the front of the party. Be sure to remind him of that sometimes. A barbarian that gets charmed by a wizard. A druid that scouts ahead in beast shape that unluckily falls into a hunter's animal trap. The point is that they should all suffer at some point in their adventuring career what it means to be in, "the worse situation ever," for their PC. That's what old adventurer's tales are made of after all.
Also, don't be afraid of giving them the occassional cake walk from time to time. It's okay if they mow through 20 ordinary guards to reach the final boss encounter in a tower. It can be important for the characters to really feel their superiority over the common soldiers, as long as they know that the boss they have to deal with is anything but ordinary or common. But even better than defeating the boss, is having the boss escape and challenge the party later.
Be sure you are using potions or magic items if your enemies have them available. I don't know why, but I feel like a lot of newbie DMs forget this a lot and it makes no sense to me to find a wand of magic missiles that went untouched after defeating an evil wizard.
If you have questions about a specific encounter, let me know. I'm about half-way through the adventure for my players.
You should add foes with different combat roles.
For example, if the PCs are up against hobgoblins and you want to ramp up the challenge then don't add more hobgoblins, add some wolves. Half the hobgoblins provide ranged attacks, half provide tanking (stopping the PCs getting at the archers) and the wolves run around as high-mobility skirmishers, taking attacks as opportunity allows (and possibly harassing the PC casters and archers).
For example, if the PCs are up against zombies and skeletons then add a ghoul or two, who starts dragging a PC off when the PC reaches zero, with the intention of killing and eating them when their death saves fail…
You should also add more interesting terrain.
A fight with cultists is dangerous. A fight with cultists inside a building they just set on fire is dangerous and memorable. A fight in a building on fire with some townsfolk on the second floor is dangerouns and memorable and tense, because the players now have to make a hard choice. Rescue the townsfolk and let the cultists escape? Ignore the townsfolk to make certain of killing the cultists? Fight the fire or let the building burn down?
A fight with gnolls is dangerous. A figtht with gnolls while behind them a gnoll bard goes to town on drums (giving the gnolls some sort of bardic inspiration) is dangerous and memorable. Think of the Warboy guitarist and drummers from Mad Max Fury Road. 
Thanks for the great ideas. Elemental Evil has been far more intense than the previous published campaigns, at least in my opinion. The party has been running from 3 out of 5 encounters; which is new, I've never seen them run at all.