I find it hilarious that Seru wasn't "supposed to be killable". Well, the players who killed him certainly didn't know that he wasn't "supposed to be killable" or they probably wouldn't have bothered trying. If Verant wants to make a mob truly invulnerable, it can do so, be it by giving it AE death touch... or simply making it a thing you can't attack like bazaar merchants.
The players are just playing the game that was provided to them. Don't punish players for not being able to read the mind of the game designer and divine which tactic is "right" and which is "wrong". If something is allowed to occur in the game world, most people are going to assume by default that this is intentional, unless it is an extremely obvious bug (spontaneous generation of currency by hailing the banker nonstop, or whatever). Guild encounters with high level mobs that they may have little firsthand knowledge of shouldn't be subject to such scrutiny. How the hell would UberGuild#1 know how hard Seru was supposed to be for them? Or that the tactic they were using wasn't just as acceptable as the various tactics that are used that take advantage of quirks in terrain and mob scripting in various other high level encounters?
Enough of this "cheater, cheater" B.S. please. Until Verant starts issuing mob killing handbooks (Vulak for Dummies! Only $19.95!) and assigning referees to guild raids, players should be allowed to assume that they can use whatever features of the game and its environment that prove the most effective, and if Verant doesn't want it done that way, they have ALL the authority and ability to make the neccesary changes, and instead of threatening to ban people or casting aspersions on their character PERHAPS they should take responsibility for their design flaws and simply fix the problem that they themselves have created.
(... Actually, just see my post below, I already ranted about this, but I had to take another moment to laugh at Verant and their invincible Seru.)