In all seriousness OP, I understand your pain. I would have had Endure done twice to only have had someone lag out. Once in the middle of the third set with 30+ lives, and once in the last set with 3 waves left. On my third attempt I finally got it. You're just going to have to suck it up. I recommend doing it late at night when less people are hogging up bandwidth for your Internet provider and making sure that there are no other obstacles. For instance the second time a guy lagged out because his sister answered the phone and his DSL cut out on him. It is kind of a shame that these are the biggest obstacles to getting this achievement but you just have to deal with it.
If possible I recommend having people over and doing it over system link (while still connected to Live) to cut this obstacle out if possible. I would have had it done this way over my friend's house awhile back but our fourth person was bad and died too many times.