How are your fighters getting an AC of 24? Full plate and Shield is 20. Where are the other 4 points coming from?
Plate armor,
+1 heavy steel shield, +1 Dex, and (sheesh, it's right in the portion of the post you quoted!) fighting defensively. I didn't even make an attempt to optimize ... a 5th level fighter can reasonably get a substantially higher AC, making things even worse for the troll.
Why would any damage be spread, ever?
Because the fighters aren't idiots. If wounded and at any real risk, an individual fighter will pull out, and maybe use his composite long bow. If the troll follows, fine ... it takes nasty AoOs and -- because it moved -- can't possibly rend.
This is basic D&D tactics, along with things like doing Aid Another to give +2 to AC to the wounded fighter. If a group just
allows a monster to do what it wants to do, well, yeah, PCs are gonna die. A lot.
Seriously, I'm shocked by the lousy tactical imagination displayed in these replies. Anybody who thinks they can kill one of four 5th level fighters with a standard troll, and wants to put some money on it, let me know.
EDIT: I do want to apologize for sounding snarky. I just truly am puzzled, because this matchup -- a troll versus four fighters -- illustrates practically a
Kobayashi Maru scenario for the troll, unless the DM seriously tilts things in its favor by means of terrain and the like.
As an exercise for the reader, and further illustration: Assume a 40-foot by 40-foot room and the four fighters on troll. The four fighters can all but guarantee that the troll will
never have an opportunity to rend. How?