Warning a bit of spoilers.
There are 3 dourhandsm that attack you immediately. fine. Then there are 4 more that are busy scribbing some notes, that you can attack. After you kill these four, then the Arnmoth gets annoyed enough to charge you. Be warned, he will call for reinforcements often in the battle, so if you arent careful, you can get swarmed over by dourhands. Group is really prefered to deal with him.
Thank you, Turbine, for listening and giving us an opt-out of FE! Good work!