Everyone piles up under her rear end.
Engage and Burn.

AE tanks grab any adds that swarm in. Ignore them and burn the named.
The named burrows into the ground. Have OT's or scouts pull in all the groups of centipedes that spawn. Burn them all and once they are all dead she will unburrow. (While burrowed, she is immune to damage and heals herself so burning the centipedes fast is important)

Rinse and repeat.

She will red text pick one person every 45 seconds or so and spit on them killing them. Where they were, a pile of goo will be on the ground. Everyone needs to get out of it as it will put a uncurable nox dot on you if you dont move.

Once she gets down about 20% depending on how long it took you to burn her down, she sort of enrages. There is no message, but you will know it happened because people will start dying like a fail mechanic has been triggered. At this point, slow your DPS because Hexapola currently has a damage shield on her that reflects a portion of your damage back on to you. Slow and steady dps and no massive hits or you will kill yourself.

Good Luck!