Honestly just don't bother playing the NES port at all; it's absolutely wretched. Literally any of the computer versions will be a much more satisfying experience. But if you must, I borrowed some strats from Yogidamonk's world record speedrun.
- Choose Compassion for the extra Dexterity.
- Dismiss your two allies to (slightly) speed up the game. We won't be doing any fighting anyway.
- Go SE from Iolo's hut to the magic shop. Take the An Tym scroll on the counter and buy two more from the shopkeeper.
From there we just need to gather up the following items from the following locations
- Serpent's Hold: Skiff from Siegebreakers, Skull Key from Kristi
- Paws: Sheet Music for Stones from Kenneth, Lockpick and Blue Potions from provisioner
- Lord British's Castle: Magic Carpet and Sandalwood Box (requires lockpick). Might also want to grab the Chain Mail in the chest inside LB's secret room for a bit of added safety.
- Trinsic: Talk to Sindar to learn the Word of Power for Shame
- Buccaneer's Den: Talk to Sindar to learn about the Grapple, held by Lord Michael
- Empath Abbey: Get the Grapple from Lord Michael
- Jhelom: Talk to Goeth, get the Word of Power for Destard
- Stonegate: Get Scepter. Requires Skull Key and Lockpick.
- Go through Destard, get Lord British's Amulet in the Underworld. Do not attack the Gargoyles guarding the entrance; just lure them away and use an invisibility potion and the magic carpet, give the code word to the guardian and enter.
(You do not need to get Lord British's Crown, do any shrine quests or destroy the Shadowlords as in the original PC versions.)
- Go through Shame to reach the center of the underworld, fly through the barrier. I strongly recommend you use the Tent to SAVE before entering.
- Once inside, IMMEDIATELY pause the game and use an An Tym scroll so the Shadowlords don't instantly kill you. Go up north, dispel the barrier with the Scepter, and go through. You may have to pause and use another An Tym scroll in the midst of this; they wear off extremely fast. Once you're clear just make your way through, once again with the flying carpet and an invisibility potion. No traps or enemies will ever touch you.
- At the end, enter the building and use the sandalwood box on the mirror for the win.