I've been pondering this a bit, since I'm running H3, Pyramid of Shadows, and have a group who are slightly ahead of the level curve, and thus plowing through most encounters.
Some spoilers for H2 and H3:
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In general, I've realized that I have no problems with them smoking a bunch of encounters, as long as they feel somewhat threatened in the 'boss encounters'. So I've generally been adding a bit to those to keep them challenging, usually making them one or two levels higher than the party, while letting them clean house on many other fights - which still drain resources, so make for a decent build-up to the big battles.
One specific concern of mine is that H3 has a fighter with a Dragon - an Adult White Dragon. Now, the group has had some hints they will fight it, and seem excited. It is meant to be a big encounter, I've bulked out its horde a bit to make it more rewarding... but as written, it would likely be a pushover. It is a level 9 solo, against a party that will be level 9 or 10 - at best, it would just be a standard challenge, and I'm looking for a little bit more.
It is especially on my mind because of our experience in H2, Thunderspire Labyrinth. In one of the rooms, the party performs a ritual to open a doorway. When they perform the ritual, however, they first have to survive a bunch of traps opening throughout the area and a Young Green Dragon showing up.
My group (as I was playing then) decided to find a spot safe from the traps where we could buff up and fight the dragon. Our cleric, meanwhile, stayed seperate to keep the dragon busy while we got ready. Just as the rest of us finally all gathered... we noticed the cleric had already more than bloodied the dragon on his own. We walked over to it, and took a round of attacks, and it died. Now, it wasn't meant to be a supreme threat - it was a solo 2 levels lower than the party - but it was still somewhat anticlimactic for a 'dragon fight'.
So I decided I wanted to make the Dragon fight in H3 a bit more intense. I didn't want to just level up the Dragon a bunch, because then it becomes annoying to PCs who have trouble hitting it. Instead, I'm adding in two extra elements. The first is a host of ice bats (element-swapped fire bats) and ice bat minions (which turn into icicles that impale the PCs, effectively ending their lives to become a status effect that deals ongoing cold damage). These are mainly just meant to be a distraction, and keep the party somewhat on their toes.
In addition, I added in an Eidolon (reflavored as a Rimefire Eidolon), since I've added in some background as the dragon having almost been revered as a god by various beings. An Eidolon is a very interesting combatant. It doesn't really interfere directly, but insteads moves into a defensive stance where it gives bonus damage to ally's melee attacks (thus making the melee brute White Dragon much more threatening), while attacking enemy's who kill its allies (like the bats). It is itself difficult to harm, since it gains significant resistance to damage and lashes out at those who attack it.
All in all, this will only ending up raising the level of the encounter by 1 (since some of this is being accounted for by the presence of more PCs). However, my main goal is to make it a more memorable encounter - filled with decisions as to whether they should clear out the bats, or deal with the Eidolon first, or simply pour everything into the Dragon to take it out as soon as possible.
(Other, less mechanical changes: I've made the Dragon Huge, just to make it more imposing on the field of battle, and provided the party with a bit of background on it, including a name - Rimetooth - and history, so they feel a bit more of a connection to it.)
I suspect, given the party's level, that they won't find it too dangerous a fight - they have several healers, and aren't likely to lose anyone in the battle. But I think they will feel much more threatened than they actually are, and that they will like they truly earned the victory when successful.
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