Use the Powder Kegs from Budger Blackpowder to weaken his carapace to make him vulnerable for a limited amount of time. Each keg destroys a third of his carapace.
A good idea on this boss is to spread out around the cavern, so the entire party is not hit by Pyroclastic Shot at the same time. Heroes and henchmen will need to be flagged repeatedly to prevent them from clumping together.
Life Stealing can be used to kill Cyndr without removing its carapace (e.g., Touch Rangers or Vampiric weapons). This is probably the easiest way to kill him. Since there is a resurrection shrine nearby Cyndr cannot regain all the life stolen before the player party get back into battle.
Cyndr cannot be hexed while the carapace is up.
When the carapace is down, Cyndr doesn't attack or use spells.
Cyndr is much easier to kill with a group of players, because the chance of a lone person carrying a powder keg all the way to him without being hit by Pyroclastic Shot is very slim.
However, killing him with heroes and henchmen only is possible: if one uses all 4 flags (3 for heroes, 1 for the henchmen) to spread their party members out in his range, one can run from Budger and to Cyndr without being hit. (impossible)
Perhaps related to Maw the Mountain Heart, or perhaps "Mountain Heart" might just be a title conferred to the biggest wurm in a given mountain.