Area of Effect, Add Target and Target X attacks are treated exactly like they are for players. Unless otherwise stated by a card, the multiple attacks cannot be done on the same enemy because they all come from the same ability. If a card says that multiple attacks CAN be done on the same enemy (as an exception to the rule), the monster will attack its focus until it's dead/exhausted, after which it will follow the focus rules to find its next target.

If a monster ability card has multiple attack abilities listed in different lines, they are considered separate abilities and will therefore be used on the same target until its dead or exhausted, after which it will follow the focus rules to find its next target.

[Errata] Note that the wording on Page 31 of the rule book regarding monster multiple attacks should read "If the monster can target multiple figures with a single ability..." instead of the "If the monster has multiple attacks..."