That's similar to an idea I had. Since we know what size party is recommended for a mark, you could apply a stacking buff on the monster that decreases incoming damage but also decreases the amount oh damage needed for earn contribution. For B-Rank, every 5th person engaged to the monster is -5% incoming damage, but lower the amount needed for contribution by a little to compensate (calculated by when you arrived, so the starting party keeps it's initial contribution at the normal rate and then everyone accumulates at the lower rate from then on). The stack grows as more show up, so the monster dies slower but people arriving at the end still get a chance to get something. This also makes zerging much less practical while making the mark actually seem like a challenge.
