If any of you haven't done this before or is simply having trouble clearing some of the dungeons (lvl 90 especially) on NG+, there is a way to farm Mars Stones (Strength) and Vulcan Stones (Constitution).
Simply do the Lyphia & Evelyn's questline until they they tell you to head to the "Sealed Darkness" dungeon in Yusof Riverbed. In this dungeon, there will be 4 mobs that will keep reviving until you kill them with Lyphia & Evelyn's special attack. Each kill however will grant you a growth item (specific to the one you're killing). Equip Ful's Mil & Ful's Tear on the character you want to start farming on & farm them for 40-45 rounds before you clear the dungeon.

If you are able to kill 1 per character (The mobs have have 500hp & prevail I), you should get 4-8 stones each turn depending on if you are using double action. All in all, this should make Mylen Ploa & Yuela turn into HUGE tanks that are almost un-killable maxed leveled & maxed prevail. Good luck & hope this helps guys.

Not bad, I'll probably try this when I get the chance. By the way, you know a good way to level up characters starting at mid-50s? Most people told me to equip with Mercy and keep whacking at a monster (I'm currently grinding at Mifu Dragon since it gives the most exp atm). I was wondering if there is a faster and less tedious way.

Doing the NG+ dungeons levels you pretty fast. I got Yuela, Emilita & Mylen to level 99 without touching a single 99 dungeon, but did about 2 NG+'s (Missed the Asmodeus questline on the first NG+). I think the fastest way to get to 60-65 is to go to the 50 dungeon on Roka's quest and on the first map, farm the shellfishes/ bird of prey by spawn camping the portal- they give A LOT of exp even until the early 70s. Another mob I would farm when you level up a bit is the dark kankuri from Sin Gate; they should give you about 50% or so per kill until level 71.

There is one disadvantage of continuously doing Deadbeat .
You don't get much of a kill count or increase to Weapon Mastery.
It's ok if you're just looking at leveling or if you're using characters with a good Weapon Mastery level.
Weapon Mastery is something that's worth working on because you need Rank M for the Ultimate Weapons.

For melee characters I spent a lot of time in Demonic Banquet (Yuidora Mine, level 70) because you tend to get about 6 kills per turn.
For ranged I used Troll's Lair (Dijenel Borderland, level 23) because that's 2 kills per turn. Bit of a pain but you only need it for a few characters.

Once you get to a high enough level the best is Parallel Rivers (Phema Sluicegate, NG+ level 90).
It's the only one of the NG+ dungeons without a side effect that can hurt you.
Plenty of high level kills so good for the kills and Weapon Mastery and you can work on 3 characters at a time.

there an easy way to increase weapon mastery/proficiency.
For example, equip a weapon that does light damage and continuously attack the wisps in Peaceful grounds (kill all the other monsters, leave 2-3 wisps alive, the char with holy weapon damage and maybe a char at home base to be safe) -> easy training since even if you don't kill or do actual damage, you still get points your weapon stat.

Deadbeat dungeon for rushed and easy levels. Yuidora Woodland for easy weapon training. If you char can't equip a weapon with holy element, just search for a monster that has absorb towards what element you can use.
Deadwolf - dark attack
Zel Lamia - ice attacks
Zombie goble - dark attacks
Tainted earthman - dark and earth attacks
Will o' wisp - holy and wind attacks
etc

As a side note to the Deadbeat King, having the item "Training Bracelet" (not quite sure where I acquired this, maybe someone else can help with that) equip it, this makes it so you cannot kill the king with any attack, as they will all stop damage at 1 point of hp. He is defeated for the rest of the current play through, so this accessory will allow you to level without worry. As a bonus, this makes it so any damage reflected at you can't kill you, since it's your own attack.

The Training Bracelet (Mercy skill) is a reward for completing a route.
You get an new one every time you start a new game using the save data.