What to know
Seal has 400 health and spawns in the Deep North.
It takes extra damage from fire; spirit does nothing to it at all. Blunt, slash and pierce land at full damage, so that is what to bring.
Of everything craftable, 
Frostfire Crossbow, 
Thunderblood Crossbow and 
Frostfire Sledge put the most damage through what it resists.
It always drops 
Seal Pelt and 
Seal Blubber.
Spawn command
With devcommands enabled, this spawns Seal in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Seal · all console commands and IDs