Any tips for must-have player cards that are a big help? My mate only has the core set, Mirkwood cycle & some of Khazad-Dum cycle for his player cards so a bit limited.
Are you both populating your decks out of that same pool? I'd think straight-up Journey would be tough enough without going to Nightmare version.
With Dain on the table, the two Hobbit saga expansions will buff up your dwarf decks without even changing heroes. But for Gladden Marshlands in particular, Thror's Map (Hobbit: OHUH) to make it the active location and Ghan-Buri-Ghan (Flame of the West) would work wonders, I think. G-B-G gets willpower equal to the threat of the active location. Those are both Lore cards, but if the Glorfindel mentioned is lore Glorfindel, you're already playing Lore, and Ori (Hobbit: OHUH) or Bifur would make a nice replacement for for Glorfindel. Thorin Oakenshield would be a fine replacement for Leadership Imrahil with Dain on the table.
With Armored Destrier (Temple of the Decieved AP), Dain can defend and ready, which is great for counterattacking.
It seems odd that you've made two tri-sphere decks, why not have Eowyn/Gimli/Thalin and Glorfindel/Imrahil/Dain to keep spheres together?
You could always go semi-nightmare by adding a resource to each hero before setup .