I've seen this guy twice while leveling in Garlaige Citadel, where both times a high lvl alliance of 70+ all /anon came and killed it. They seem to be a linkshell who has killed this HUGE HNM many times because this thing is a pain. And I mean PAIN if you're leveling and Serket decides to spawn. So here's my 2 gil on how to get through without taking too much of a setback.
First of all, Serket's area attacks have a very wide range. Also, as many have already posted, the poison (20 HP / tic) will take out a lvl 50 party very quickly. Luckily we had a bunch of high lvls who knew serket well who stood next to us, doing nothing but Poisona, over and over. If you are the healer in the group, take charge and get your group far as you can and if your party get's poisoned, Poisona them all.
Second, If you DO die, which is rather likely, get raised AFTER Serket is dead, or if you get raised already/ have re-raise on, don't "activate" raise until Serket is finished off. Otherwise the nasty AoE or Poison will kill you instantly. I watched one of my party members die three times by doing so.
And as a last note, if you are an LS or group of people planning to kill it, give a quick /shout or two explaining what you're about to do, just so parties can be alerted and move away if they don't want to die.
I'm not saying this HNM is impossible to live through, I'm only noting what my party experienced, and sharing Serket's nasty attacks.
Lastly, this HNM does not aggro, and spawns every 24 hours.
Hope this helps!