These mobs are uber-tough and frankly they need to be or it wouldn't be worth the effort of even showing up. However it would be nice if they could be properly tanked but even lv 75's are dropping off. True if they were in a party with a semi-competent healer they would stand a better chance, but if the lag or hits are too strong, you die. And if not raised before the end you lose w/e you have earned even if you waited 2-3 hours for them to advance and arrive, or stay dead and if possible get a portion of it. (Thats even if a raise lets you keep your points which idk of yet.)
And as a Pld I know it's our job to voke and death has our number on speed-dial, but no Tank is going to live more than 2 minutes unless they have a great party, gathered on the same mob working together instead of everyone running solo blasting away as a Pld/blm using aoe nukes. (gulity

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Like Skaw suggested and many others have agreed, gather a good support party around you like you would for any other mission/quest/Xp, if you are a regular at Besieged you know from checking your region info you have plenty of warning from a lv1 preparing to an actual attack. Try to get your friends and linkshell involved, it's fun and nothing to lose but time (well gil depending on w/e potions equipment you buy for it) and it doesn't have to be treated like some random ballista you just showed up for. Make a lv 5 besieged static or gather an alliance and all kill the same mob.
I don't blame other Pld's for not voking though, not sure how quick it is for a lv 75, but @ lv 60, i press my ~Voke~ macro, it's 20 seconds before i see it even went off, another 20 seconds to see I've been hit once, then I'm dead looking at near full hp waiting for lag to catch up and tell me how many hits it takes to get to the gooey filled center of a paladin pop.
Let's work with the system however we can until they change it to make it better and then figure out how to work with whatever they just screwed up now.