Give the hag the ability to go invisible with a bonus action, that will spice things up, and give her some minions. Most save or suck spells require vision, so invisibility means they can only target her with attacks or AOEs.
For terrain, I'm not familiar with the module, but I would include some of the following terrain features: a chokepoint (doorway, bridge, etc) where the hag could have a minion block the way, hiding spots/cover, and maybe a terrain hazard like a swamp with tentacles that harass anyone who comes near (can be defeated but more of a passive threat).
Did you already do the fight? How did it go and what terrain elements did you include?