Now that is a hard question.
Weapons, shields, amulets, orbs, and even resonance should be considered
Everything has its pros and cons, so it is not easy to compare the possible options.
Obviously the stronger you are, the easier it is to survive.
The strongest weapon is 毘沙門天の宝塔(Bishamonten's Pagoda), leveled up from 落し物(Lost thing).
It has a whopping 40 base attack, but it is not good for survival, because it uses a portion of Reimu's HP.
The next strongest weapons are 祓串, leveled up from 木の棒(Wooden stick) and メテオリックオンバシラ(Meteoric Onbashira), leveled up from 丸太(Wooden log).
They both have 30 base attack.
丸太 has a chance to break completely when attacking, but it can destroy walls and traps. If you can manage to get 2,000,000 exp. to the orb and level it up to level 5, it will never break.
木の棒 gains a base orb at level 5, which will sometimes activate an attack spell card without using your Ps. Quite useful, but be careful not to attack Rinnosuke.
The strongest shield is the 神守大二重ラセンスキマ(Great Duel Spiral Sukima protected by gods), leveled up from スキマ(Sukima).
It has 30 base defense, but it still breaks when flung at walls.
There is another shield called アンチスパイラル(Anti Sprial) which also comes from the スキマ(Sukima), and it is very tiresome to make it, but it doesn't break.
The almost obvious amulet of choice would be the 博麗アミュレット(Hakurei Amulet). Just put in the orbs and it's done.
For other amulets, 天狗の古地図(Tengu's Old Map) is the best choice you can have. It tells you the locations of enemies and items. You can search for potential MHs or stores.
If you can get hands on the 大輪の向日葵(Large Sunflower) flask item, you can 'mix' some items to make the 四季酒「幻想郷百花繚乱」potion item, which has the 【花】orb.
This orb, when on a amulet, will give Reimu +50HP, +25 stomach, +3 Max Attack, and a chance of strong hit.
For the orbs, it will be good if the weapons have 【必】,【治】,【多】, 【広】.
必中【必】(certain hit) orbs can be found in 草刈り鎌(mowing scythe) weapons, and gives Reimu perfect accuracy with 10% reduced power. Leveling up the orb reduces the penalty.
治癒【治】(heal) orbs can be found in はたきのようなもの(duster-like thing) weapons, and heals Reimu proportional to damage dealt. Leveling up the orb can heal up to 40% of damage dealt.
多段攻撃【多】(multi-hit) orbs can be found in 猿田彦味酒「幻想風靡」(Sarutahiko Sake 「Illusionary Dominance」). It is a result of a long mixing sequence.
It gives Reimu the power to hit multiple times with reduced, like Aya. Leveling up will give you more hits with a bit less power. Lv. 9 needs 6,100,000 exp., but gives a total of 20%*10 hits.
広範囲攻撃 【広】(wide hit) orbs can be found in 妖刀「犬走」(Youkai Sword 「Inubashiri」) weapons. It is made by synthesizing the right orbs in the right order in the right weapon.
If the weapon has this orb, the range will increase to the three tiles in front of Reimu. Leveling up will widen the range to the sides then the back sides.
The shields would have 【地】,【重】,【吸】,【避】
地雷名張 【地】 orbs can be found in 三日月プレート(Cresent Plate) shields, and reduces the explosion damage down to 60% of Reimu's HP. Really important. Or else, stupid luck can kill Reimu easier.
重石【重】(heavy stone) orbs can be found in 漬物石(Pickle Stone) shields, and prevents Reimu from being flung. Its most important function is to block Yuugi from throwing Reimu into traps.
吸血耐性【吸】(anti-blood sucking) orbs can be found in 紅いリボン(Scarlet Ribbon) shields, and prevents Remilia's ability.
回避【避】(dodge) orbs can be found in 触角リボン(Whisker Ribbon) shields, and makes Reimu dodge attacks more often.
変換【変】(change) orbs can be found in 絵本(Picture Book) shields, and changes status to damage. Leveling up will reduce the percentage of damage. Good status will also be counted into damage.
This is all I can think about now. These are written under the assumption that you can bring items to the dungeon.
