First we need to know about the game and party. I believe the book is based up version 3.5. If this is correct, clues like Mialee's Magic Missile spell creating 3 bolts tell us she is level 5 (possibly 6). Krusk uses Rage twice in a day; so at least fourth level. And Vadania takes tiny animal forms like the normal falcon and squirrel, so she is level 11. About Lidda's level we can not be sure. I think it reasonable to assume three of these characters have been adventuring together so assume Krusk, Lidda, and Mialee are all fifth level. And extremely poor; as evidenced by their lack of quality armor or even the most basic scrolls or potions. Krusk, for example, has AC15 (leather, no shield, DEX, and maybe 16 with Dodge feat) and Vadania does not have a magic weapon . . .
So our Troll shows up in camp and its average damage vs Krusk is almost 25 (chance to hit times average damage plus nearly 1 in 2 chance to Rend each melee round) per round (i.e. Krusk dies in round 3). Just for comparison, Tordek
based upon the NPC section of the 3.5 DMG beginning page 49 would have an AC21 and take an average of 11.5 per round from the Troll. Quite a difference. Vadania is weak both in offense and defense. Where is her animal companion? It would be a significant source of additional damage as well as a meat shield to absorb some damage. Also some brown bear(s) from Summon Nature's Ally 5 (6) would not only provide more damage to the Troll than Vadania, Lidda, and Mialee combined it would also absorb considerable punishment from the Troll.
Vadania is rivalled by Mialee for incompetence. Mialee casts one Magic Missile and then apparently decides less damage is better an uses her staff for an average of less than one HP per round. To sum up the battle. Troll entered camp. Krusk rages and attacks. Others attack. Round 3, Krusk dies. Round 6 Vadania dies. At this point the Trolls regeneration exceeds the scratches supplied by Lidda and Mialee. Round 8 Lidda dies. Round 9 Mialee dies. I'm not considering the level 1 Cleric significant.
Consider if Mialee uses 3x Magic Missiles and a Burning Hands scroll. Round 3 Krusk dies. Round 4 Troll dies and does not regenerate. Quite a dramatic difference. And with a Nature's Ally or a proper tank the party might have survived without any death.
The players are incompetent RMT's and the author should never have written about a subject of which he is so ignorant. I do not recommend these books.