Behemoth, Odin etc. are just far too easy. Only those who are already in the area are lucky to get a sniff at them before they die, anyone else who hears about it in linkshell or company chat often always arrive just in time to see the corpse. Of the times I managed to get there in time there's so many people that the boss disappears and then you can't get credit for fighting it because you can't see it to attack it. Even if you are already hitting it it'll disappear and you'll then lose your lock on it. This is just stupid, they need to be phased and take longer to kill because on my server they're dead within a minute and all you see are people spamming aoe in a hopes they're damaging the boss and complaining in shouts that they can't see it.
