The combination of the Blue Gale bow (guarantees every hit will be critical) and the Dismantle skill (critical hits break whatever body part they hit). It doesn't work on bosses, but for most encounters, it allows an archer to tear through the enemy, killing them in a few hits regardless of their defense and the archer's attack power.
The game also includes a few deliberate Game Breaker weapons, provided you overleveled a few plot-important characters before the points where they leave the party. Of course, the levels they need would have broken the game anyway, so this is possibly just the game's way of compensating you by providing your remaining characters with another way to break the game.
The "First Aid + Metabolizer (or Ring of Healing) + Survival + Lifeforce Blessing" combo allows any character to became near invincible. The first two allows you to recovery half of the damage inflicted by an enemy attack, the Survival skill makes a character survive a otherwise fatal attack with 1 HP and the Lifeforce Blessing Sealstone doubles healing, and therefore allows you to recover the same amount of damage received by an attack.
Anybody familiar with the original game knew Odin was a Jerkass, but this game revealed that he had a plan to use Bahms' soul as replacement for the Dragon Orb to prevent Migard's destruction, but opted out of it and decided to allow Migard to be destroyed just remove any knowledge of the Sovereign Rite outside of Asgard. Even Freya is shaken up by this.