What to know
Baby Seal has 200 health and spawns in the Deep North.
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 
Lightning Strike put the most damage through what it resists.
Spawn command
With devcommands enabled, this spawns Baby Seal in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Seal_Pup · all console commands and IDs