The Wood Woad / Mephit fight: Somewhat spoilerish, so:
There's a back way into that area if you have enough Misty Steps or really good jump distances. It allows you to get your party on top of the plateau before starting the fight, which allows you to rain he** from above without worrying about your party members. Fly the camera around to find it.
There is also another method for the Mephit fight, as well.
Soon as you enter the Bog, go left and go to the end of a dock....
Jump left from the dock onto a tiny island....
Move north toward the tree and jump to another rock which is on the main land with the Mephits.
Target an Ancient Mephit from hide/sneak and once battle commences, jump to another rock, fire, wait your turn, then jump onto a tall/high rock and continue taking out the Ancient Mephits.
You can pretty much wipe everything from that tall/high rock and they won't do anything but move while your taking everything out.
One solo toon/character is all you need...may take a little bit but you are safe in that position and literally can take your time killing everything, just make sure to target and take out the Ancient Mephits first (4 of them), as has been indicated already by others.