Tough one, lacking an enemy max HP option as well as AOE HP burn... that's hard to get around.

Several good options for weak affinity tank (Skull Lord could be an interesting one, assuming the other obvious candidates Sepulcher and Geomancer have other uses already), plus some other good champs like Deacon or Sethallia, but without a Coldheart, Royal Guard, or AOE hp burn option, it'll get very difficult. 

Unless you can just prevent the spider from taking any turns at all... Maneater, Armiger, Deacon, Lua, plus Skull Lord? Not sure honestly, it might not be enough TM reduction (can't test it as I don't have an Armiger built myself). 

Hmm ok I'll give it a shot :)

Its really hardd..

Thanks!

Armiger, Deacon, allure (turn off her a2/a3), and you might be able to control the TM.  need a solid tank and a healer though.

zepher sniper cheese might be an option?  you can cheese it with paragon too, but it's a bit more complicated.

You could also try mashaled for aoe true fear and leach, and then use deacon for turn meter and dec def armiger for turn meter geo mancer for tank and posible dec acc and the 5th could be like ninja for freezes on spiderlings or what not or 5th could be a suppourt champ like man eater or zelotah they key will be to make sure the main spider takes the least amount of turns as possible so maybe throwin in allure or high khatun for more turn meter reduction its going to be tough for sure your missing a lot of key componets to beating spider 20