Assuming I have no clue about forging, how would I go about acquiring an Odachi with Change to Attack (Stamina)"? Ideally with A+ but anything'll do I guess. I never did any of the forging stuff back when I platinumed the game in February.
With the caveat that I don't know if the new DLC & game version changed something, I'll take a shot at this. All of this is assuming you don't already have the ability as an inheritable trait on another weapon and don't want to try to revenant farm for this particular project.
First, it is pretty important to determine if you really want that A+ or not. The better scaling specials can only appear when the weapon is created / looted initially and can never be replicated via inheritance or reforging. If you really
really do want it, then you are stuck forging the same type of weapon again and again until you get the ability you want. Expect copious amounts of copying you backed-up save file to recoup your forging material losses (expect to do with anyway at some point, as reforging can be pretty damn brutal as well).
When forging new items, it is kinda important to consider the base item itself, if you want to get the most out of your gear. For one, even divine and ethereal items get "only" 5 random specials, so having at least one pre-set special, like some elemental damage, or the weapon being in some set, is recommended, though this also can mean being stuck with inferior elemental damage values (generally speaking, there are two "pool" for the elemental/poison/paralysis values and quite a few weapons are stuck with the pool of lower values). With two or more pre-sets, you can just concentrate on getting the one or two specials that only appear upon creation. With only one, or worse, no pre-sets, you'll generally want to forge an item that doesn't already have an inheritable special (the ones marked with an icon that looks something like this []>[] ), so that you can add one later via inheritance.
The actual forging itself is pretty simple, as the game will by default go with the highest available material quality. Pay attention to the tooltips though. To get divine items, you will need to add divine fragments. Just before you press the big ass 'Forge' button, you press triangle to insert the fragments. The more you invest, the higher the chances you get a divine one, but you need to consider your total amount of materials and divine shards to determine how many you want to put in for each round of forging. If you're short on shards and have plenty of other materials, it might be better to just put in one shard as you still get something like a 40% chance for a divine. On the opposite end are the 80+% chances for a divine with 10 shards. I tend to go either min or max investment on that front.
Once you've gotten the beatstick you're going for, assuming you didn't try to get the A+ version, the rest is up to the reforging process. Your weapon can only have a single Change to Attack (insert stat name here) -special in it, so you'll definitely want to re-roll those away if you're gunning for a specific one. They are still relatively rare, so you'll want to use divine shards when you're rolling for that one. The new Tempering option can cut your time in this regard, so if you have a little luck and multiple grades of Umbracite, you can be spared from a great deal of headache and save-scumming.
Now, this post pretty much covers only the part you asked about and only dips into inheritance and re-forging which will be required to max out your beatstick. Xuchu had a really good post on the late game damage output optimisation just a while ago.
