Hmm, isn't part of the reason that spider has lots of health because you can make it take about a third of its health in damage from destroying the webbing it's standing on and having it take fall damage?

When I fought ti with my friend, we made sure to do that and it went reasonably well. I assume if you were baller you could run onto the central webbing, making him come attack you there, slowfall yourself before destroying the webbing and get a full kill along with a casual fall into the underdark. (haven't tried that yet because my party was just a fighter and ranger on that run)