A few things to add, turn your graphics on balanced at least so you can see the swarm and where it is.
The Scarab incubation, as Vasan said, is a debuff x5, that is on a 25 or 30s duration. If you not dispell all 5 debuffs before the timer expires, bugs will spawn and heal the boss. Best bet is to have both healers curing the people as they call it out in vent. If this fails, get ready to AoE the spawned bugs quickly to minimize healing done to the boss.
When he hits 50%, he will continue to do the Scarab Incubation as well as a new Death Swarm. This is the same root that was happening when you were running down the tunnel at the beginning. I would suggest all grouping up very tight around the boss, in the back corner. This allows you to simply mash the "Help Ally" button and not have to run around and un-root people. The boss has no AoE's or cone attacks that I know of, so get close.
Un-rooting your people is key, as a swarm will spawn at the entrace to that room (where you entered) and will move towards your group if the "Death Swarm" (root) is active. So dispelling it quickly is key. If the swarm touches anyone they instantly die. So kiting the boss to the other side maybe be necessary if you are not doing a good job "Unrooting" people.
This is a DPS race, the boss will berserk after a while and hit for 30k. So speed is what you need here. And again, making sure those bugs don't spawn by curing the x5 debuffs will make the boss not heal, and will make the fight much, much easier.
I would also recommend 1 tank, 3 DPS and 2 healers. The boss doesn't hit very hard.
Wif Blud,
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Dexteram Tzeentch