This boss is a single phase fight and has several abilities to watch out for and a rather tight enrage timer of 8 minutes. Overall, about 16.5k DPS is required to beat the enrage.
There are 4 friendly NPCs being affected by Flash Freeze and hence frozen in ice block at the start of the encounter. They cast helpful spells during the fight once freed from their ice blocks. The exact details of these NPCs can be found on the Hodir Wowpedia page.
Begin the fight by freeing the far right NPC (the Mage) from its ice block. Hodir will immediately engage. There is no need to free the other NPCs at this point as the mage will do this. Once Hodir engages, the player will be affected by a stackable debuff which deals frost damage over time. The debuff will continue to stack as long as the player remains standing still so it is vital to move around in order to prevent this debuff from stacking too high. Additionally, moving does not remove all the stacks immediately but only 1 at a time. Fortunately the friendly mage NPC can conjure a toasty fire which eliminates this effect completely.
Hodir himself has several abilities that requires the player to continuously move anyway. Icicles will continuously fall from the ceiling at the player's location that deals 10k damage plus knockback. They can be avoided by moving away. Hodir will also use Freeze, which places a dispellable root effect on the player. This is especially annoying when Hodir uses this while an icicle is about to fall from the ceiling.
Hodir's third ability is Flash Freeze, it is a 10 second cast time spell that freezes the ground. Anyone standing directly on the ground will become frozen just like the NPCs at the start of the encounter. Fortunately around 7 seconds into its cast time, a large snow mound will drop, and the player can then walk onto the snow mound and avoid the Flash Freeze completely. Unfortunately the friend NPCs aren't as smart and they almost completely ignore this snow mound and hence they will be come frozen and need to be freed again. If under any circumstances the player is frozen by Flash Freeze, the game is instantly over, at least from a soloing perspective.
Hodir's final ability is Frozen Blows. In a nutshell from a soloing perspective, this is simply a 10second damage boost. During this time, his physical melee hits is reduced by 70% but he will deal an additional 30-40k frost damage with each melee hit. Use a cooldown to live through this....or just simply Flash Heal. Another way to mitigate this damage is to gain distance from the boss. The only way to do this (without run speed pots or rocket boots) is to stand under a falling icicle and use its knockback to gain distance.
There are 4 friendly NPCs throughout the fight. The spells they cast are all useful in some way but the 2 most useful NPCs are the Mage and the Druid. The mage, as mentioned earlier, conjures a toasty fire which removes the raid-wide stacking debuff from standing still. The mage can also place a debuff on the boss that increases spell damage taken. The druid casts beams of light at various locations on the ground. Standing within those beams of light provides a haste buff. It is recommended to refresh dots while standing in those beams for extra DPS as the enrage timer is tight. Additionally use the time while he is casting Flash Freeze to catch up on healing, as well as use SW:D occasionally as mana is somewhat tight as well.