Skell Knights have a considerable amount of health and will take a few hits to down. Skell Knight Bosses are also above average. Skell Knights, like all "Knight Class" enemies won't attack until attacked.
Skell Archers are immobile with generally average stats. The regular ones will fire single arrows in the direction of the player. The Bosses will fire a volley of three spread shot arrows.
Necro Mages will fire magic in the direction of the player. They can also spawn Skell minions that are fast, but can be downed with a single attack.
Zombies are quite annoying in that they can respawn one time, gaining back half of their health and an increase in speed. They are decent fighters with above average stats.
The Skeleton King is a pretty intimidating foe. To start, he will jump towards the player and execute a single attack. He will repeat this one time before attacking extremely fast for about a second or two, then will slow down a bit. Right after, he'll start his minion phase where Skells constantly spawn, and he attacks at a medium rate. He then starts at his first phase again.