seriously, i have everyone at 99 (minus stat capped chars) and those are just no. are there any tricks or magic items that allows me to not get one shot and do more than 1 damage to whoever goes into prevail?

Blood of Revival and your own characters with prevail. Yuela with Prevail III + Undress III becomes unbeatable late game.

how does yuela get undress?

Enhance her God-Slayer clothes, it's somewhere along its upgrade path.

ok will try
also i saw around people saying "stats are not a problem because fountain of youth and transmigration", what are those?

1. Finish yuidora mines first
2. get as much ultimate weapons as possible (you should get around 80-90% of the weapons, the missing weapons will be most likely the one's who need the yuela route only accessory and the girl which route you have open[if this is not your 3rd+ game])
3. farm the yuidora mine map with all the goblin to get money and upgrade the armor of every girl
4. snipe as many unpleasant stuff as possible with emelita, sewari, hakuren, elizasleyn, mylen and w eushully(or any other with 3+ range skills)
5. oh fuck every lvl 90 map is easy as hell even without using fountain or stat increasing stuff

... The 90 dungeons aren't so hard that you need ulti weapons. And honestly after the first one you pretty can do all of them. Also stock the hell up on Bloods of Revival and Breaths of Elyun, as well as large potions. There really isn't any reason to have less than 999 of them before entering a map.

I just womped ass at Advent Bay with Mylen, Wil, Yuela, and a few of the other decent characters and revived to hell because they die often. It's going to happen. Advent Bay was the first one I did and I was amazed at how easy it was. As long as you use the characters who can actually kill you should be able to do fine.

managed to clear mirage forest, now with being lame and deactivating spwan points with portal stones i might even go for the 100%

however fuck advent bay, see you next playtrough with normal

for mirage forest i just give my powerhouse chars the amulet to get earth resist >S and they get little to no damage from turn damage

What I did was make portal stones (Magic Stone + Darklight Flower) to seal the monster portals and then use Eushully's Enhanced Battery (9 range, they can't attack back lol...except in Advent bay, almost every monster there has a reflect skill -.-) to kill all the monsters

all monsters have projectile defense which is more effective more far you attack them from, hell it's 50% less damage just from 3 tiles, how could you harm them at all?

new game with normal opposed to hard and suddently advent bay 100%

Advent Bay is probably the easiest of them all - half of the enemies in there can't do much damage (Seraphim, the spirit)...
Give Emelita Prevail 3, Soul Shield 3, and Succubus Embrace to kill the angels with Projectile Defense. You can also put someone with Emelita's Guardian next to the target (not her!) to shield a hit. Or just use Stone Golem with the Demon Gloves and Prevail to bulldoze through everything...
The only real threat in there is the Seraphim's Reflect, so don't use Prevail characters to fight them and you'll be fine, since they have bad defense and do crap damage even with Prevail active.

The hardest level 90 dungeon is probably the desert level, since your choice of characters is severely limited by the damaging terrain, and every enemy in there has Projectile Defense with Cover and Prevail.
Emelita with Prevail 3, Soul Shield 3, and Succubus Embrace does the trick. (The enemies have lower magic defense than physical.)
Yuela and Stone Golem with Intimidate can help too. (When Intimidate triggers, all Cover and Guardian skills are ignored.)

what? mirage forest is the only one i managed to clear before realizing i was playing in hard mode

however cool for the intimidate thing, it will make it less painful to grind for drops

Equip your characters with Weapons + Death Skill.
Prefer those who can do range attack (as far as possible)

Wil : Carbonel's Hammer (Holy Element ,+Death)
Yuela ,Mylen : Naqua Ninja Blade (Normal Physic ,+Death ,~30 Evade)
Emilitta ,Elizasleyn : Kamic Staff (Holy Element ,+Death)
Ragsmuena : She has Vorpal Counter ,most of her weapons +Death.
Black Eushully-chan : Black Putettle (Dark Element ,+Death)
Miss Eukleia : Black Petal Fan (Dark Element ,+Death)

Maybe Suina : Bowling Ball (Dark Element ,+Death)

Change your weapons when face mobs strong against those Elements .

all 2.0 dungeon mobs are marked as boss thus can't be instakilled

Really ???
I don't look at them careful enough to realize that point .So fail ,me.

I used Stone Golem to do the desert for my first bonus dungeon. Was hard fought with a fair amount of revivals and a lot of stamina potions but hey nowhere else that you need them. BUT this dungeon has a weapon for stone golem that has bloodsuck 3. So after this is practically invincible and the other dungeons become much easier.

Bloodsuck and Prevail don't work well together, especially on someone with high HP like the Stone Golem.
And TBH, Bloodsuck is not a good skill at later levels, because it overrides many other passives, like Flurry and Pierce, preventing them from triggering at all. It's designed to keep Sharty alive in your first playthrough, but by the time she gets 3 (at Lv.50), you'll probably be on your last mission already...