IME with a CR 5 troll against 4 5th level Fighters, typically the troll will kill one before it goes down.
Well, we're getting into specific tactical talk here, so I won't take it beyond this post, but ...
IMO, if four 5th level fighters (say, straight from the DMG, but equipped as PCs) can't handle a troll without one of them dying, they're being run by idiots.
For the fighters, assume an AC of 24, 42 HP, an attack bonus of +8, and damage of 1d10+4. (This would be a sword-and-board fighter. AC down to 22, damage up to 2d6+7 for a two-handed fighter. Neither of these are anywhere near optimized. Because this monster does so much damage if both claws hit, the fighters are fighting defensively, and flanking.)
The troll hits with a single claw (+9) 41 percent of the time, and with both claws (and thus a rend) 9 percent of the time. It hits with its bite 5 percent of the time. Average damage (ignoring crits) is about 7 per round. Average damage -- assuming a rend -- is 38. Barring a crit, one of our fighters will survive the average damage of the troll even if it hits with everything, and hitting with everything would be very,
very lucky for the troll.
Meanwhile, the fighters hit the troll's 16 AC 65 percent of the time, for an average (barring crits) of 10 damage. Due to regeneration, it will probably take 8 to 9 hits to bring the troll down, which is three to four rounds of attacks, assuming one fighter drops out of the battle after taking serious damage.
Looking at the percentages, it should be pretty clear that it would take fairly horrible luck for a fighter to die before the troll goes down. With average luck and good tactics, the fighters will all still be well conscious, with maybe --
maybe -- 40 damage spread out among them.
So, is it
possible for the troll to kill a fighter before going down? Sure. But it's very unlikely, and
certainly not typical, unless the fighters use no tactics or bad tactics.
In the interests of disclosure, these numbers aren't precise, because I'm doing all of this in my head and probably screwed something up. But they're close enough to get the point across.