Asdivine Hearts II

The weapon upgrade system in Asdivine Hearts 2 works identically to Fernz Gate, where you can synthesize other weapons into a "base" weapon to increase it's level (e.g Sword +50). You do this by using the Custom option in the menu, selecting the base weapon, then the ones you want to feed to it.

How long will this take?
This is the "grind" of the game, but not too bad compared to other RPGs. It's on par with other Kemco/Exe-Create titles. It took me about 3-4 hours in total, but I also didn't realise for ages that I could increase the difficulty of battles to make the gold grind quicker. So it could be dramatically shorter for others.

How to do this
You will need a LOT of gold to purchase all the swords to feed to your base weapon. (I needed to max out my gold counter nearly twice over).

While I'm not sure if this is the most efficient method, this is the way I farmed Gold (and EXP at the same time:

Once I was around level 100, I set my Zack/Felix team with the following rubix and skills:

Any team should work - but I left it as Zack/Felix.

Rubix
-- Acceleration Jewel (so they always goes first)
-- Gold Jewel (Gold earned up by 20%)
-- Metal Jewel (Higher chance of metal enemies)

Destruction spell
This is a super OP unison spell that hits around 50/60 times, and makes the metal enemies go down easily. You might not get the critical first try every time, but the odds are pretty good. The unison spell requires the combination of Dark Matter/Mini Big Bang
-- Mini Big Bang is a Void magic spell
-- Dark Matter is a Shadow magic spell
These spells can be learnt by equipping the Shadow Jewel α and Void Jewel α, which are both available for purchase at most end-game shops. I'd get these equipped ASAP on Zack/Felix once you get them, so you can learn the spells quicker.

Grinding areas
There are actually a lot of options for grinding areas in the game. I used the "Mushroom" islands below Arnolt temple on the Asdivine world map, but this topic on the RPG Insanity forums has a bunch of areas:
https://www.rpginsanity.com/forum/asdivine-hearts-2/19723-go...

Bump up the encounter rate in the menu to double, or use an Attract Jewel (battle every step) if you have one to make the process quicker.

Basically - as soon as the battle starts, cast Destruction with the Zack/Felix team, which will generally wipe everyone out instantly. The metal enemies will often max out your EXP for the battle, and you'll go up 7-10 levels at a time. You'll also get a shitload of Gold - which is what we're after.

Once you can easily beat the enemies, bump the difficulty up to something you're comfortable with, which increases the rewards and makes the process easier. Some battles on Expert would net me 7 million gold each time.

How to level up your weapon
I didn't spend a lot of time finding an optimal way to do this, so I'm happy for anyone else to chip in with a more efficient method.

Once I had max gold, I would bulk purchase the Oblivion sword from the Forgotten Village weapon shop, and upgrade Zack's weapon with all of them at once. There's diminishing returns on how many weapon levels you'll get each time, but this thankfully caps out at 999 weapon xp. Basically - after that 85 Oblivion swords would get me about 10 weapon levels a time, at a cost of 2.7 million gold. Don't forget that you can use the Select All feature to select all your feeding weapons at once. Also - sell off any unwanted weapons for other characters to give you extra inventory space to make the process quicker.

Then it's just a matter of Rinse, Repeat until you run out of money, then farm again to get enough to come back and repeat the process.

If you have spare Holy Stones, you can also purchase gold multipliers to make this go quicker, but they're expensive. However, personally I used my stones for Secret Presents and Treasury Keys instead.