Hiya!
Here's my "reasoning" (from a DM stance...as that is what I am)...
Players: We keep heading down the trail towards the coast.
DM: After about an hour of wandering this trail, it weaves around the more rocky terrain, always descending at a leisurely grade.
[rolls a d20] Hmmm. Suddenly, from both sides of the trail, out pop a dozen goblins!
Players: Crap dogs on a stick! Wasn't anyone looking?
DM: Their Stealth roll beat the highest Passive Perception...sorry... Anyway...uh...Megdan the Mage is dressed in robes or travel clothes, right?
Megdans Player: Yeah, traveling cloak and robe.
DM: So...AC 11. Got it.
[rolls 12d20] Megdan takes 35 points of damage.
Megdans Player: ... ... Well, that puts me at an imaginary -15.
DM: Oh? Hold on then...you have 20hp? Then you take 20hp, drop to 0, then take another three hits which are automatic criticals, so you are actually...uh...dead. Again.
Players: Run away! Run away!
DM: Ok. Rennie the Rogue has leather armor, right? AC 14 was it?
Rennies Player: Yup.
DM: Ok, as you all flee back the way you came, you all move 60'. So, let me see how many arrows hit Rennie
[figures out how many goblins are in close versus not close range...works out to be half...rolls 12d20]. As you all flee, Rennie is hit by 8 arrows, the last one was a critical. Rennie takes 45 damage...which, iirc, means he drops at 30..meaning he takes 10 and 20 under 0. ...Oh...uh...guess he's dead too.
A bit extreme? Only by a
little bit. Mages and rogues would be dropping like flies. Hell, anyone
other than the guy with heavy armor would be dead by the third room in the dungeon. If the DM used his/her 'logic' to RP.
What I am advocating is that the DM use his/her 'logic to RP', but with a strict eye on the way that 'logic' would be in a D&D world. I mean, do 100 orcs just camp outside the town and charge in at dawn? Yup. Why? They are orcs and that's what orcs do. What would make more sense? The orcs
not camping outside the town and instead hide nearby until night...say, 10'ish. Then sneak as best they can into town and into allyways, barns, stables, etc...wait for the signal horn...and then start kicking in doors and killing. But they don't. Not "normally" anyway. Why? That's not what D&D orcs do.
If the DM played "every" monster as his/her logic would dictate to be the 'best' way to kill the PC's...then we'd have no sense of different monsters being, well, different. An orc's "logic" will be quite different from a kobolds, which will be different from a hill giants, which will be different from a beholder, etc, etc, etc.
That said, no matter the creature, the DM needs to think of 'logic' in terms of D&D. And in D&D "logic", when the DM RP's three of the 5 orcs ganging up on the full-plate fighter, it's because that's what a D&D orc would do. Is it smart? Probably not...but it is 'honorable' to an orc to kill the tough guy over the weaklings. If they all gang up on one guy, after that one guy is dead, they can all fight over who did the killing blow.
^_^
Paul L. Ming