If you simply want the enemy to approach and attack and nothing else, then - assuming you are using an evented battle - you set your event
Autonomous Movement box (bottom left): select 'approach'
Trigger: select Event Touch.
For the commands,
Battle Processing and select your troop, or select Same as Random Encounter
Decide if you want to allow escape and if so what happens if they do escape (e.g. the enemy sprite moves away from the player for a few tiles to give the player time to decide what to do next).
And that's it.
EDIT
You also have to decide what you want to happen to the enemy after the battle.
If you don't want it to reappear ever, then Self Switch and a second page conditioned by that switch with nothing on it.
If you want it to respawn if you leave the map and return, then 'Erase Event'.
If you want it to respawn if you leave the whole dungeon and return, then you will need a switch e.g. Dungeon 4 ON which conditions your second page, and then on your transfer out event, have Dungeon 4 OFF