There are two versions of this powerful creature: A mobile one, which (like the Sand Wurm) is burrowed and will pop-up when players least expect it and follow players around and engage them in melee. Then there is a stationary one (that does not burrow), which will use the devastating AoE attack Wurm Siege.
All Siege Wurms are easily stopped by a ranger with distraction skills. Concussion Shot is one of the better options as Wurm Siege is a spell, the Dazed condition allows interruption of all its sieges, leaving the rest of the party free to finish it off. Note however, that conditions such as Dazed last only half their duration on these Wurms.
If someone stands on a bridge while it is under the bridge, it will stand still and cease to do anything.
Bug! Sometimes, if you run away from mobile wurms while they are above ground, then run right back to it, it will cease to do anything except continuously activate Wary Stance.