eamon
Explorer
To be sure, I see that argument for some monsters (essentially; those are design errors). However, the monsters' attacks are clearly labelled as being melee, ranged, area or close by means of symbols, and the PHB specifically addresses melee attacks that target multiple enemies in the grey box on p. 270:Every group I play with lets the 'make a melee attack against three targets' acceptable for marking purposes, so it makes some things easier in that respect. I do wonder how much of WotC thinks one way or another on that ruling, though. I'm pretty sure certain monsters would have been designed differently if they'd been thinking about it that way.
Targeted: Melee attacks target individuals. A melee attack against multiple enemies consists of separate attacks, each with their own attack and damage roll.
By contrast close attacks talk of an area of effect which may contain multiple targets and share a damage roll.
So if you get a bonus (or whatever) on your next attack, then that bonus applies to only a single melee attack but possibly several targets of a close attack. And if you're marked, then the mark penalty is separately evaluated for each attack individually.
So, while there are monsters where the usage of multiple attacks doesn't make sense (rule 0 those), in the general case it's all rather explicit: multiple melee attack targets are resolved as separate attacks. And in the hydra's case, that makes perfect sense: that hydra really is making fully independent attacks with the advantages (he can focus fire, PC imposed penalties to a single attack apply only to one target) and disadvantages (marking) that implies.