guys any tips for third route? The game suddenly become so tedious that every normal enemies is like 1 billions hp up even on normal...
I've been playing the 3rd route on paradox difficulty, and what I did was first fight Uriela using Est (in grand noah). Uriela has no way of damaging nightmares, so just stack as much damage as you can on Est and she can win the fight on her own. Then, once you've beaten Uriela, she joins your party temporarily.. but you don't actually need to do her quest (she leaves after the quest is finished) - you can just leave her in your party and go off doing whatever else in the world (she also has some funny dialogue when you do the quests in the other cities with her in your party), and she's quite strong at that stage of the game. I rearranged some of her abilities and adding the supercharge abilities and after doing San Ilia's event you can get a better job for her, and I got her to a point where she could do 100 billion damage per turn to all enemies or so with the true hero song doubling her stats (more/less depending on the enemies).
You can also similarly keep Sariela in your party at that point in the game by starting that quest and then leaving once she joins your party without finishing the quest, but I didn't find her as strong as Uriela (certainly not in terms of doing damage anyway - she does have a prety funny ability which resurrects her at the end of every turn when she dies which might have some kind of utility in the future, but it's not immediately very useful anyway). After that I did some grinding and explored the world a bit - with Uriela being able to one shot the early enemies even on paradox difficulty you can level up jobs pretty fast.
Later on when I stopped relying on Uriela and advaned the quests I got Luka to be a complete glass cannon, using an accessory that makes him start with 20% health but gives +50% stats (that accessory doesn't count as a stat boosting accesory), along with a +50% attack accessory, and stacked as much +damage from gems etc. onto Luka and used the strongest weapon I had, and used abilities that give Luka buffs when he's at low health (especially the one that gives a 2nd action). Luka does something like the talk skill that gives him 3x actions on the next turn (4 with the crisis extra action), full power for +300% attack on turn 1, Lucifina uses daystar to help keep him alive, someone else uses true hero song, and on turn 2 Luka is doing 4 tirllion+ damage by spamming his strongest sword+hero chain with priority from the quick sword ability (can be more if the enemy gets some of the status effects that make him do extra damage, 4 trillion is when they didn't get any status effects)