Well, this was how I did it... I might be wrong, though.
(I'm using PK edition, so it might be a little bit different.)
In database, go to Monster Group, and place whatever monster you wish to place there.
Then, for the first page, place trigger as Turn 0 (first box.).
This will cause the event to trigger on the start of the battle.
From there, show battle animation if you want to show attack. It can be found on the third window. You will also need Force Action and Deal Damage for this.
The code went something like:
Show Battle Animation[B]: [1.<Enemy Name>], [Slash]
Force Action: Hero 1, Attack, Last Target, Execute Now
Deal Damage [1.<enemy>], [<damage>]
To use it in a game, just use Enemy Encounter and use that certain battle.
And the code continues. I don't really have the time to explain with screenshots. I need to study for a test tomorrow.