If you play solo honor mode then you are already know that you need some important items always ready such as scrolls, potions, elixirs in sufficient quantity. And items like Silver Pendant and Shapeshifter's Boon ring. So, when you are going to fight Minthara ( lethal or non-lethal doesn't matter, you still have to defeat her one way or another if you want Dammon's stuff later ) here are some tips for non-lethal cuz i like to have them all in my team and i find it interesting and extra challenging for solo honor run especially for spell casting builds. I did it with a blaster sorlock.

Before the battle make sure that you have 5 lvl at least, Loviatar's Love buff from Abdirak in goblin camp, wardrums are destroyed in priestess Gut location
and all goblins near the priestess Gut (including her), infront of the prison with Halsin and near the spider pit are already dead. Dont kill Dror Razglin before Minthara. Take a spot from a high ground using stairs or scroll\potion of glorious vaulting and destroy the war drums where Minthara talks to a goblin.
This positions are safe for destroing the drums and it shouldn't cause a fight but if it somehow does then exit the fight using Potion of Invisibility, move a few steps away from your last position, end your turn, wait.
You also can throw a few hp potions on the ground so you can take its effect without action in a fight later.

When you are ready to attack, equip the Helmet of Autonomy (for resisting Hold person spell just in case or another one you like, its in the Underdark before Bhooal location) + Safeguard shield (ONLY with proficiency so you can cast spells, Dammon sells it.) buff uself with using Oil of Sharpness on Phalar Aluve and any +1 ranged weapon if needed + Elixir of Hill Giant Strength (you now can hit things even if you are a spell caster with no weapon proficiency) + hp potion with Whispering Promise ring buff (more accuracy and resistance from bless) + Phalar Aluve buff "SING" for moar accuracy and saving throws and recast potion\spell of glorious vaulting, Feather fall (Just in case if the fight takes place on a high ground), False life (or another source of temporary hp), Guidance (Silver Pendant), Shapeshifter's Boon ring effect.
These items also can help you with passing performance checks for Loviatar's love buff effect.
You can use Mirror Image and Blur just as well.
Now you have extra accuracy, some protection from Hold person and another crap. All that stuff you will find before going for 3 goblin leaders anyway and if you are familiar with solo runs then you know that all that buff setup is important everywhere.
You dont have to be proficient with any weapon to cast spells so it will work with any spell caster build. Keep in mind that non-lethal works only with melee non-elemental damage. These items take that into account.

After using all mentioned buffs, start a fight from afar with an arrow of Ilmater so she cant heal herself every round or wait kiting and dealing damage from a distance without using special arrows until she spends all of her Lay on Hands charges (4 in total). Sometimes she doesn't heal herself more than once.
You need to kite her by using buffed jump or Misty step far away from her so that she spends all of her lvl 2 spell slots (2 in total) on Misty Step instead of Hold person (if she casts it and you fail = rip most likely if not resist on the next turn). This is also the reason why we kill priestess Gut and goblins nearby: for a further fight space. So no one can interfere.
If she cast Abjure Enemy on you - misty step.
This area is fine for kiting and dealing damage.
You can block stairs with a chest on top of it. It may force her to use Misty step or run to the next staircase chasing you.
So bring also some Misty Step scrolls and Potion of Glorious Vaulting. Attack from afar and jump away.
Once she is out of 2 lvl 2 spell slots (she used 2 misty steps) and 4 Lay on Hands, toggle "On" Non-Lethal attacks in your passives if needed, lower her hp to ~15 hp and below and then strike with Phalar Aluve in melee (green action) and pommel strike (orange action). Or use haste potion and common attacks. Leave the armor on her, it may come in handy to her in the second act when you are going to free her because 2 gnomes that guard her can use Blindness.

At the Moonrise Towers Prison she can be freed in several ways.
Pass a dialog checks in the prison, but it can be hard without inspiration.
Or lure her guards with minor illusion cantrip or using Conjured Cat with her "Meow" action to a chasm near tieflings cells and drop them there.
If you don't have these spells, then close the door of this room and break a bottle of water between two of her guards (so they can be vulnerable to cold and lightning for 3 rounds) and strike with Scroll of Ice Storm or Scroll of Lightning Bolt. Preferably from stealth to surprise them.
You also can drop a weapon for her so she can aid you in a fight with these two gnomes.
Once it's done give her a Potion of Invisibility so no guard will bother you and escape. Watch out for Scrying Eye, it can reveal invisible.

Yeah, if you are at 5+ lvl in any martial class, not a spell caster then you can simply prebuff all the stuff, drink haste potion and hit first, but if you don't defeat her in 1st round or not run far enough after first hits she can use Hold person on you. You have some protection from it but i don't want to give her a chance to cast this spell.
Why Phalar Aluve ? Its a +1 finesse weapon (can be used with DEX and STR) with a Bless effect ("Sing" in sword actions) for 5 rounds, it gives you protection, accuracy, and you can use it along with Safeguard Shield even if you a not martial class.
The whole concept of this guide is to make it doable with every build.