A ranks should be scaled to a party of 8 people as a base

Whenever anyone else gains enmity on the monster, it should gain a damage reduction buff that stacks up to 99 or more. The more people the higher its resistance.

Yes there is griefing that could occur, but really, with the metric F-ton of people who want seals, is that really going to happen much?

They could also make the buffs wear off over time until a new person gains threat on it to reset the time or in the event of griefing, have it so if you don't do any damage over a period of time the monster loses the stack they contributed.

This could also in that sense be a debuff effect to the attacker instead of the monster. IE:" More then 8 people have attacked this monster, your damage to this target is lowered." the debuff would read.