Just increasing the defense/hp may solve the problem on high-pop servers where there are armies of people killing each and every hunt. But it gimps the small party that happens across an A-rank during off hours on a low-pop server.

A better solution would be a semi-scaling hp depending on how many people that was in combat with the mark. If more people join during combat, the monster's max hp would jump up but the monster would stay at the same % current health before people hopped in.