Overlord II

The easiest way to get this is if you enslave Nordberg. The population will begin working for you: mining, smithing, farming, etc. In the town there will be a fenced in brown spot with several people farming...apparently baby seals. This creates an infinite spawn of seals. The easiest way to find it is just to listen for the seal barks.

In the north of nordberg close to where you have to use the blues to pull the raft there is a brown pen on the waters edge. The seals here constantly respawn two at a time.
Just put your minions on a gaurd marker and leave them there and they will keep killing all the seals that spawn.
In the mean time you can go off and either enslave or kill all the townsfolk.

There are two ways to achieve this achievement, throughout the game you must go through Hunting Grounds where you will find several of these baby seals, you can use this place to kill them leaving and entering Hunting Grounds but this is not the best method, the best way is in Norberg Town there you must enslave some humans where they will be working for you, north of Nordberg there will be some slaves who will make equipment and forge weapons for you, there you can see there will be some of these baby seals, go to this place with your minions and press right in the middle to leave your minions on guard, now just let your minions kill them for you.

After conquering Nordberg, I made two trips to the arctic coast area. Use the wolf-mounted browns to kill seals. Before the second trip was done, I had 100 seals.