Story related and can't be missed.
The following has minor plot points for the DLC.
To avoid the annoying nexus power puzzle, side with Janeka (The Mage) initially. This path is easier and will not require the use of youtube. Then once at the seal you will have another opportunity to take sides. This time side with Larius (The Warden Ghoul). Doing this makes Larius kill Janeka and avoids an unneeded fight with Larius.
The fight with Corypheus (The final boss of the DLC) is a bit of a pain. I played a warrior so please adjust your strategy accordingly, but this should help with the general idea of the fight. My Rogue never stayed alive very long in this fight, so I just worried about myself, my tank and my healer. In order to make him vulnerable to attacks you will need to interact with the 4 statues to break the source of some of his power. Every time you do some enemies will pop out and attack. Defeat them before moving on otherwise your party members will suffer (I tried running ahead). Make sure to dodge and take into account Corypheus' attacks while you do this part. They work in a rotating fashion, so just go with the flow when moving to the next statue to interact with. There are 4 statues to interact with to cut the flow of power. Once you have stopped the flow of power from the statues he will leave the middle allowing for close range combat, his attacks are deadly so pay mind to them. After taking so much damage he teleport to the other side. Chase him down and continue wailing on him until he returns to the middle. Then deal with the 4 statues again. The environment in which you are fighting will be become more volatile and harder to traverse as the fight continues. Slowing down the fight.
Overall this fight isn't really hard, its just drawn out making it annoying to fail.
Have a lot of healing items and keep an eye on your party's health. Especially your healer. This fight is drawn out and will require your attention the entire time.