Drexthar Bloodtwin plus 4 Healer/ Reviver 

The best team is the champions you have that can kill it of course!

If you are having trouble bring a strong cleanser, avoid champions with a lot of aoe, avoid turn meter manipulation champions as well.

GL

Post your roster so someone can try and help out. :)

You definetly need a good cleanser, with a short cooldown. Reliquary Tender is probably one of the best champs in this fight.

There are different ways to defeat the Nether Spider.  I personally use a niche strategy I invented on my own, I steal the counterattack buff from the small Spiderlings and kill them after that. They heal the big Spider only when they do a counterattack, so it's no problem when you've stolen or removed the buff. The Nether Spider puts out one poison per Spiderling when it summons new ones, but with a cleanser that's not a big problem.

You could try other ways, for example the typical Spider-strategy of this game with hp-burn on all enemies, but here are only 4 Spiderlings, not ten as at the Dungeon Spider.

It would be really good to see your roster to advise on a strategy.  Also, are you talking about normal DT or hard?