Moving to VX Ace Support
In this case, you're wanting specific "how to" instructions, which might vary with the different makers.
I use scripts to change difficulty level (it increases or decreases damage done by enemies and party members, so pretty simple). Here are some non-script options that I can think of off the top of my head:
Use evented battles (ie - monsters show up on the map and have a Battle Processing command, rather than using random encounters), and having a variable set to the level of difficulty, and then have extra monster events that are conditioned to only appear when the variable contains a higher number (higher level of difficulty). This doesn't mean the monsters are more difficult, just that there are more of them. Or have one monster sprite represent a troop of several monsters, and have multiple pages on the event, conditioned again by the variable, where the battles on each page are against different-sized troops.
To change stats ... maybe you could have states that enhance the enemy stats, and at the very start of battle (on each troop page), add states depending on the level of difficulty variable.
Or give enemies some skills that can only be used when certain switches are on (turn on switch X for medium difficulty, which gives them SOME extra skills, and turn on switches X and Y for hard, which gives MORE extra skills).