there should be an enemy that adapts to your play style. if you are constantly hitting it and those around it with ranged weapons, it should grow ranged armor. if you are slicing and dicing, it should obtain melee armor. this would prevent you from just playing a single style, you would need to have the ability to switch on the fly.
The only problem with this is that "classes" will have enough equipment so that you can't switch so easily. I'm assuming that this also means the combat styles as well, in which case it would be a bit hard to pull off the switches. I suppose the added challenge is the point though.
They stated (forget where, maybe check blue's list of ideas) that you canNOT change on the fly. Anyone can use any style of gear to be a "class" but you can't carry enough different gear with you or something to be able to utilize a new style of combat without first returning to the spaceship.
Probably want to read this whole thing then http://community.playstarbound.com/index.php?threads/list-of-announced-game-information.450/
What if instead of growing armor to combat melee or ranged damage, they became resistant to fire or physical damage? IMO damage types like this would be much easier to implement and it won't require a change in class just to combat them, only switching weapon dmg types.
I think the way you fight should not only increase their resistances but also their behavior,this could be justified by the creatures having a hivemind so that the information of an individual are shared to the whole species. Maybe they could also develop different tactics against you depending on their numbers:for example one distracts you with melee combat while another one charges a power full ranged attack(they could try this tactic if you use a weapon with low knockback but they would not use it if they knew your weapon has high knockback or if you have a gun that can pass through enemies!).
Now that would be something. Kill it before it get's too smart for you. I would definitely love to find something like that. At least a few times i would probably not kill it if i met it, but try to see just how evolved could i make it. I think that if both it's appearance and behaviour/resistance would change it would be just perfect. Something like this looks a bit like your idea(I found it, didn't make it!): http://m0ai.deviantart.com/art/On-t...-109620664?q=boost:popular shadow demon&qo=73