Who would have knew a red orb farming/grinding spot i used from 11 years ago would come in handy for a few achievements!
Misson 11 - Fate : offer the chalice and open the path.
After fighting a death scythe as soon as the chapter begins, you'll then have to jump down a hole into an underground tunnel directly infront of you.
Down that hole you fight an area full of blades (the lizard men with shields). Defeat them, crack the wall and defeat a few more, then proceed to exit the hole and collect the "sign of chastity".
When you jump back down the hole, the blades will have respawned. Hit devil trigger and light them up with the shotty to raise your combo to "stylish" then finish them off with ifrit combos. (kick 13 is a must have)
Doing so will net you between 200-250 red orbs, and a blue orb fragment the first time you beat them again.
Simply jump back out of the hole into the area where the last artifact was collected, then jump back in and they will have respawned. Rinse - repeat, Happy grinding! :-)
The Method above i found very useful to obtain the following achievements -
collect 10,000 red orbs.
collect 25,000 red orbs
collect 50,000 red orbs.
Purchase all of dante's skills.
All skills requires the purchase of all additional moves for both Alastor and Ifrit.
Using this will also enable early purchase of all purple orbs (to increase devil trigger gauge) and also all of the full blue orbs(increase health bar) available in the store.
Note there is also a statue of time in the grinding/farming area so that you can buy the skills ect as you grind, or just check how many red orbs you need to purchase the items.
If you are concerned with getting an S rank for the mission due to time limit, you can simply save the game after you are done farming/buying skills ect, then reload the save. This will start you off at the beginning of the mission again, but now with all of your new skills!
Comes in very handy for when you have to start the new game + on higher difficulties.
Hope this is helpful! Be sure to leave a comment if it is, and if it isnt - feel free to leave some constructive feedback!