Even though Rotscale looks exactly like a Bone Dragon, which are undead creatures, Rotscale itself is not an undead creature. Therefore holy damage does not inflict double damage and it is considered a fleshy creature (i.e. can suffer bleeding and other conditions). However, the bone dragons and other undead surrounding Rotscale are still susceptible to Holy damage.
Rotscale uses its skills with a halved casting time and recharge, approximately double the power and like all Tyria bosses, hexes and conditions cast on it only last one half of their normal duration.
Grenth's Balance is a great tool in making the fight against Rotscale shorter. A player needs to raise their maximum health (using skills like Vital Blessing), sacrifice or otherwise lose health to a very low level (using skills like Blood Ritual or Infuse Health), and then use Grenth's Balance on Rotscale. This will drain a sizeable amount of health from him. Using 2-3 back-line bonders and Symbiosis you can raise a tanks health above 3,000 - this allows a massive Grenth's Balance spike when the tanks health gets low.
Use enchantments carefully. Rotscale uses Desecrate Enchantments which does more damage the more enchantments there are on the targets.
Protective skills that limit damage are valuable against Rotscale's increased damage output. Examples include Protective Spirit in the monk profession, bonding as well as protective spirits in the ritualist profession like Shelter.
Dazing Rotscale will greatly diminish his threat. One simple way is to use condition transfering skills (such as Plague Touch) to transfer the dazed effect caused by Deafening Roar back to Rotscale, although a more reliable and consistent method is to use the Ranger skill Broad Head Arrow. Combine the daze effect with Frustration for a faster kill. Also consider combining with Epidemic or Fevered Dreams to Daze the entire enemy group.
Another good strategy for fighting Rotscale is to have a Ranger maintain Winter and have a necromancer continually keep Spinal Shivers on him.
Because a resurrection shrine is nearby, it is possible to defeat Rotscale with only henchmen in a fairly short time. If you attempt this, it is best to kill each of the Rotting Dragons first. Usually it is possible to destroy one before your party is killed, making subsequent attempts easier. After all four Rotting Dragons are killed, attack Rotscale himself.