|Odin is arguably the worst unit in the game and is used, mainly as Blárblade+ and Moonbow fodder, but a few more recent additions and the fact that he is one of the most common heroes to pull making him one of the easier heroes to +10 means that he is not totally worthless, and this interesting build makes use of his unique strengths, which is really the only way to make use of him, because if you try to build him like any other mage in the game, he will, invariably, be an inferior version of that mage.
IVs: We're going to want Odin to be +Def or + Res depending on whether you'd rather use him to fight bow/dagger wielders, or tome/staff wielders better. Taking a bane in HP is recommended only because he can't really afford to give up anything else, dumping speed might cause him to miss some important breakpoints, and dumping attack means he has even MORE trouble doing damage since, unlike staff or dagger users, he doesn't have a strong debuff to excuse low damage output. + Res potentially lets Odin run Ploy skills in his C slot without them being unlikely to trigger whereas +Def gives him more staying power if something gets into melee with him.
Weapon: Blárserpent+ Is a new weapon that perfectly synergizes with Odin's mixed defenses. Granting Distant Def 3, Odin's defenses when attacked at range jump considerably, and when synergized with the rest of his kit, result in him taking, essentially, no damage from most units at 2 range. (choosing whether to forge +Def or +Res should match whatever you choose as your boon)
Assist Skill: No particular assist skill is any more necessary for this build than any other, but don't confuse "Flexible" with "Not necessary". All of Odin's punch on this build comes from being attacked on the enemy phase, and unless his special is charged, he will likely not do meaningful damage on his turn so it's better if he can use his turn to rally or reposition teammates than take pot-shots at foes.
Special: The choice between Glacies or Ignis comes down to which stat you took as your IV boon. Odin, like Matthew, is one of the unique circumstances where we take the 4 turn charge Ignis or Glacies over the 3-turn Bonfire or Iceberg because Odin's attack is SO bad that he's honestly only ever going to do any damage at all when his special triggers. Given that, we want the turn that his special DOES trigger to pack a wallop. FYI, when counter-attacking a 2 range unit with this build, before any buffs, merges or supports are taken into account, Glacies or Ignis will be dealing an additional FORTY damage on trigger.
A Skill: Distant Def 3 doesn't add any frills, it just keeps this build doing what it does better: tanking ranged attacks. When taking a ranged attack to his boon/forged stat, Odin's Def or Res will be a staggering 50, and his non-boon stat would be a, still respectable, 43 before any buffs or supports are taken into account.
B Skill: Guard 3 is the preferred, but expensive choice here. Reason being: Odin won't be KOing enemies unless their resistance is horrendous or his special is charged, so if he's letting them start charging their special on him, he might be doing more harm than good tanking their shots, PLUS it helps stop them from piercing his massive defenses with a Luna or the like. Ironically, Dull Ranged 3 is also a very nice option for him given that it further accentuates his identity of being an anti-ranged unit as long as you don't mind cannibalizing 2 Morgans. Due to high defenses making Vantage not needed, high speed making quick riposte not often needed and the fact that he isn't a really useful attacker on player phase making desperation not needed Odin doesn't actually need some of the highest demand B-skills in the game. Odin DOES have other options such as running wrath as a niche option that could allow Bonfire or Iceberg to be run instead of Ignis or Glacies if you can lower his health to the threshold, or a ranged breaker skill to hard counter a particular unit. But, honestly, if you aren't running guard or dull ranged, it would probably be best to give him a budget B skill such as Seal Res 3, or Seal Spd 3.
C Skill: Odin's C skill is largely flexible. A Resistance boon Odin with a forged +Res Blárserpent+ will hit 32 Res before merges and buffs are considered, so Ploy skills CAN be reasonably run on him Res Ploy 3 probably being the most useful to make it easier for Odin to do meaningful damage. Other notable options are Threaten Res 3 and Threaten Spd 3 for being generally useful for debuffing enemies that end up near you after you tank them (just be sure not to double up on the same debuff as your B skill if you went with a budget "seal X 3" option), but, ultimately, as with many builds, MOST C skills are workable, and this slot is fairly flexible.
Sacred Seal: Distant Def 3 is wanted here for the same reason it's our A-skill and Blárserpent+ is our weapon. All these bonuses stack and, while it's situational and relies on Odin being attacked by a ranged unit, when he is, he's a stupidly sturdy wall, and he can return mighty special counterattacks.
~Sonic The Hedgedawg