Guardian treasures : each guardian upon defeated will leave a random set of available loots in the chest they are guarding. Same name guardian but in different place will leave different set of treasure behind. And the guardians are usually replaced by another one when you advance to next chapter so that leaves some of their items/rune/gear become missables.
Chris level 99 trick : In Chapter 1 of the Chris POV. In the first major battle between the Zexen Knights and the Lizard/Karaya alliance, you are told to retreat, and the battle is quite difficult, but it can be won. If you win, you will be awarded with an increase in two levels for Chris, Borus, Roland, and Salome.The level 99 trick involves dying after this level increase, and then repeating the process over and over until you get to level 99.
Easy Fury Rune: during Hugo chapter 3, you can encounter Bronze Sun in Ancient Highway which will drop this (Bronze Sun in other places don't drop fury rune)
Fire Magic Rings: you may want several rings in order to boost your Flame Champion fire skill to S. These rings can only be found in Azodess's chest (in FlameChampion hideaway) during chapter 3 or early chapter 4. After you can name your castle, this guardian will be replaced by Nemesis which won't drop fire rings.
Early Double strike runes : these runes can be obtained early if you're lucky enough, you will find them in Blue Mantrix's chest (in Mt Heitou). Or at beginning of chapter 5, refuse to rest when Sebastien asks you then you can go to Lebuque to stock up Double strike runes from the shop there.
Jongleur rune - Nei : if you really want to make the game a breeze, Nei is a good choice when her Chanting skill reach S, song of madness brings ~2.5x damage for everyone in party as long as Nei is not interrupted.
Pale Leather set : another game breaking solution. it heals user for 25% damage dealt on enemies. Imagine Hugo hitting 4 times on a target for 300 damage each hit, In total he heals himself 300 HP after the attacks...
Mount pair : there're total 3 mount pairs in this game, a good way to strengthen both attack and defence as well as turn order. When they attack , they deal the sum of their power, when they get attacked, they share the damage received half and half.
Unite attacks : some unite attacks are extremely powerful which can easily reach damage cap as 9999 or deal a good amount of damage to a large area with the right set up. Notable as : Futch/Franz/Fred or Emily/Sanae/Sharon or Shabon/Sr.Joe
Fortune rune : with this rune you can easily level up your character from 47 to 67 or 55 to 75."During Chapter 4 and 5, with the Fortune Rune and just 2 characters (your hero and another), a level 47 character can be boosted to level 67 (57 w/out Fortune, which is still nice to achieve),note that this game's EXP is awarded by how many characters you are in your active party, dead or alive, so Character B won't get extra EXP if Character A is KOed at the end of the battle.So anyway, go to the Flame Champion Hideaway with the 2 characters and just defeat an enemy party of 3 Copper Suns. There can be 0-2 Nariquas and it won't make a difference.
Your Flame Champion can go alone to Flame Champion Hideaway once you obtain a Fortune Rune to jump from 50 to 70."~KFCrispy from suikosource.com
In battle versus Sarah in Ceremonial site, if you use a party of 2 characters(main character included) with one equipped with Fortune rune, they can jump from level 55 to 75. If you use party of 3 characters with one equipped with Fortune rune, the 3 of them can jump from level 53 to 73.