On the issue of players being surprised and thinking it odd or even wrong that a troll fights defensively...
You should probably insult them and call them whiners because there is nothing in the troll descriptions which would lead them to....
MM180 "Trolls have no fear of death. They launch themselves into combat without hesitation, flailing wildly at the closest opponent. Even when confronted with fire, they try to get around the flames and attack." Same page... "Int 6."
Hmmm... perhaps there was a point to their surprise at the trolls acting so out of character. Perhaps its not just "low int players... yuck yuck."
Now a good GM, instead of smacking them around a bit and spinning a tale for the boards to ridicule them, might well have made the "troll fights defensively" a CLUE! When his players unanimously exclaim "WTF, a troll fighting defensively" he could have said with a wry smile "yeah, that does strike your characters as odd, now that you mention it. OK, so who is up next?" and proceeded unabashed.
Monsters need to be roleplayed too. Thats the GMs job.
Then, as the story progresses the players might well unravel the mystery of the smart trolls. Perhaps they are dominated and trained by an evil mage. Perhaps they are products of some other oddity. Perhaps they are not really trolls at all but rather some shifter-thing able to mimic trolls powers but not as stupid.
I agree with everyone else that the tricks the PCs use should be used by their adversaries but i add "when appropriate for those characters to do so."
Stupid, fearless, not-afraid of death trolls would not seem to me to be "in character" to fight defensively, especially if he was ahead at the time.
My gang has encountered lots of trolls and the vast majority of them were pretty much as described, straight ahead guys, only rarely running and then only when seriously damaged (near 0 hp) from acid and fire. When that occurred, they fled. They did not use sophisticated target selections, usually swinging at whatever is closest, whatever just hit them, although rarely they choose enemies based on whether they have fire/acid.
As such, an intelligent party often can distract the troll from one ally who needs to fall back to another healthier ally. Note, by the monster being played as a 6 int rampager, the party gets a chance to SHOW in game that they are not 6 int themselves. They can see IN PLAY a difference between normal intelligence and low intelligence. They could be TAUGHT BY ME how to make INT affect your IN CHARACTER decisions.
or i could just decide trolls int is not gonna be a factor and play them like experienced fighters like all my other opponents. I could let there be little or no difference in the quality of tactical decisions made by ogres, trolls and smart fighters or even extremely intelligent guys... they could all be just as smart as me, the guy running them, and the players would BE TAUGHT BY ME that int does not matter in how your character shouls be played.
Funny thing, the time i threw a classed troll with a brain (fighter levels and 10 int) at them they caught on right away that he was "smarter than your average troll" as he did things like move to gain cover, take advantage of terrain and use good target selection, not falling for their usual anti-troll tricks.
They knew they were in a serious fight and took him very seriously. The fact that he was DIFFERENT and made better decisions than the usual trolls made for a wonderful addition of tension to the combat. He was exceptional, and that showed. They even tried to talk to him.
I much prefer that result to the one supported by this thread.
You know, it would be easier for me to play ogres and trolls as tactically with it as trained fighters and adventurers and monsters known for their brains and cunning like dragons and such, rather than having each on react and play based on his nature, but it would seriously detract from the flavor, would serve to cheat those "smart" adversaries of their moment to shine, and IMO would serve to seriously impact the relative CRs.
All in all, not a good thing IMO and not in play for my games.
To each his own.
"Trolls are definitely too strong for a 2nd level party to be expected to fight them. At CR 5 (minor ability tinkering notwithstanding), it's a reasonable fight for a 5th level party -- a 2nd level party should have about a 50/50 chance of entirely getting wiped out by a properly run troll. (Extrapolated chance of the whole party getting wiped out by a succession of 4 trolls: 94%.)"
TROLLS are not necessarily out of the range for 2nd level parties, particularly if indeed one at a time.
The CR = PC level is not "standard" it is merely the baseline for encounters which wont kill any PCs except by a fluke and which will drain 20% or so of resources. It is not meant to be "the encounter."
Look in the DMG and you will see they break down a typical adventure to include scenarios of a wide variety of ranges of CR vs PC level. They run the gamut from lower CRs to CRs of 1-4 above the PCs to grande finales of 5 or more CR higher.
They key elements are that as you reach the higher CRs, it becomes more risky to the PCs and in many cases they are great for full strength parties but lousy for a tired party "open the door...oh crap" encounters where they are unprepared.
A single troll vs a 2nd level party is manageable, albeit a tough encounter, and one in which PCs can die since the trolls strength is DAMAGE OUTPUT.
If, as the GM described, the trolls were set up scenario wise so that the party would not encounter more than one at a time (this includes making sure the others cannot react to the fight) and have the ability and a decent reason to pull back after each one, and did indeed have warning that trolls were afoot (and an opportunity to equip for troll hunting) this is still a serious fight in which the PCs have to be smart to keep everyone alive, but perfectly reasonable.
Of course, if these are not true, if the PCs will end up facing trols in pairs or one right after the other, if the PCs while warned had no good opportunity to equip to take advantage of that foreknowledge, or if these turn out to be exceptional trolls gifted with tactical insights so they won't be victims of their normal low int or reckless assault tactics... in short, if the GM plays the trolls so that party attempts to outsmart them are doomed to fail, then you probably have more troubles for the PCs.
Enjoy your games.