Ones that:
- keep a certain distance from the player.
- keep out of the line of fire of the player.
- run in a spiral towards the player.
- swoop in towards the player and then back off.

etc.

Fixed.

- Swoopers, that try to swarm around you and are fast, but turn slowly so they end up making irregular circles around you before stabilizing into some kind of pseudo-orbit, all the while firing inwards.
- Snipers, that teleport around the map slowly (keeping long distance) and "take aim" at you (perhaps marked with some crosshair or a laser sight).

- Modular enemy. Latches together to form bigger monsters until they reach critical mass and get better weaponry and things. Needs to be hit twice; once to dislodge a smaller enemy, and then once more to destroy the small enemy. Sounds kinda like a boss, actually.

-Enemies that run at the speed of Teen Angst.

Enemies that are attracted to you when you take a certain action. A simple example would be the Boos in SMW that come at you when you turn your back, and hide their faces when you look at them. Another example would be the blind moles in my game that run at you if you make a sound, stop and look confused if they hear you from another direction all of a sudden, and dig back underground if they can't hear you at all. Another simple example of that might be an enemy that's invincible, but homes in on you when you fire your gun, as if attracted to the energy. Like the pulse bombs in Metroid: Prime. Or the lasers that zap you in Abe's Oddesy if you pray. Basically a way to outlaw a certain action in a given area if it helps develop a fair challenge for a player. Even simpler; high gravity sections that disallow jumping, or wandering energy fields that don't hurt or impair movement, but make weapons unoperable while you stand in them.

Wow, that got long-winded.

Perhaps you should have stated that in the original post. Even more helpful would be an explanation of what you mean by "rather fixed arenas."

If you've ever played Crimsonland or Robotron.

Exactly.

And based on their wikipedia articles, one is a scrolling game (Crimsonland), and one isn't (Robotron)

edit: Obviously X-G can read things in Wikipedia normal mortals cannot, since the Crimsonland article says nothing about a confined arena.

Which only confuses the matter more, as now he's creating a game that is like those two games, only those two games are different.

No need for the apologies. I was just trying to point out that the more information you give, the more likely you are to get appropriate responses.

Perhaps it could start off slowly, then gain speed momentum and eventually bounce off walls. The faster it's going, the more damage it does.