For random encounters, it wouldn't be practical without scripting. However, without having had much experience in RMXP, and not currently at a computer with either VX or Ace, I would be inclined to think this is somewhat possible with events.... although very exhausting and limiting.
You wouldn't be able to have the enemy's automatically change based on character levels. As MA stated, there are scripts (at least in VX) that allow monsters to level up with the party that offers more efficiency. If scripts is not an option for you, then what you can do is:
1. Decide how often a monster and its troops will change throughout the course of the game.
2. Create copies of the same monster and troops in the database but with altered stats, skills, and whatever else that makes them different. Do this for every time you want the monsters to change.
Note: It won't be very feasible to have the monster to change for every level up, because you would then have to create x amount of copies for each monster and troop based upon level cap. It would probably be better to just alter there stats every 5 levels or so.
3. Create events for your monster encounters (no random encounters). Use conditionals that call the troop based upon the character/party level. So if the character/party is at level 1, it will call forth the troop with lower stats, and when the party is at level 5, it will call the same troop at higher stats, and so forth for level 10, 15, etc.