Hmm, I've always found that the strategy for Nether spider boils down to 2 things:

• An AOE cleanse to remove the poisons every 3 turns when he uses the skill that summons the spiderlings 
• An AOE Nuke to kill the spiderlings when they spawn every 3 turns

If you have both of the above, then the battle should become trivial to manual as you whittle away at the Nether spider's HP each round.  It doesn't heal without the spiderlings, so as long as you can keep your HP up then it's a guaranteed win

You already have the foundation of a team to do this.  MMage will do the AOE cleanse on his A3, as long as you never use it until after the spiderlines are summoned.  You have Scyl+Doompriest+MMage to help with the healing each round, and to put decrease speed on the boss

I guess it comes down to how good of a nuker you have to deal with the spiderlines.  It looks like you are using Zargala for that, but she is the negative affinity.  You are better off looking for a magic based champion (e.g. Dark Elhain, Ninja or even Kael), but it will depend on who you have built out for raw damage.  Whoever it is should only use their AOE nuke whenever the spiderlines are summoned, and then just A1 or something on the boss in between.

I guess this leads me into my next question, are you manually running this battle or auto'ing it?

Also, don't bring anyone that decreases turn meter to this battle (e.g. Coldheart or Alure).  The passive that damages you based on turn meter reduction hits like a truck

You have Sethallia - I always found her very useful there to strip the counterattack buffs from the spiderlings, as well as her heals/cleanse of course. Though once the counterattack buffs are stripped, the spiderlings do need to die before Agreth's next turn, otherwise she will just put a ton of extra poisons on you to compensate. 

Also, if you ever get truly stuck, especially on the Hard DT, you can go for a Paragon cheese. In this case, Paragon needs a Deflection set so he'll bounce some of Agreth's poisons back onto her, needs to be booked so he can keep himself unkillable forever, his A3 disabled, then you just go in with a team, revive Paragon as he'll surely be dead before his first turn, then let him do his thing for half an hour on auto while Agreth slowly dies of her own poison. 

I use Sethalia as well.

Having just cleared 20 Hard DT Nether Spider, I would like to propose an alternate strategy that I ended up using: Just ignore the spiderlings.

You will want a cleanser to remove the mass poisons on the opening round, but after that just focus on Agreth with single target attacks, and surviving.  The spiderlings only heal Agreth if they counter attack, not if they normal attack.  Additionally, Agreth will only use his A2 Brood Feast if a spiderling dies or the counter attack buff wears off.  In the case of the latter, the use of A2 will put counterattack on all of the spiderlings again, but not trigger the group attack, which makes the damage from this skill quite minimal.

I ended up going in with a single DPS champion with a single target attack, and 4 champions built around support.  These were:

• Reliquary Tender for cleanse, 15% continuous heal and resurrect if necessary
• Scyl for 10% heal every round and 30% slow on A1 (never use A2).  Also brought a second resurrect
• Brogni for shield and HP burn on A1 (never use A2).  Removing the counterattack buff from the spiderlings is bad as it makes Agreth's A1 attack multiple times
• Spare slot - I tossed up between Doompriest for heal every turn and debuff removal, Ursala for strengthen and resurrect, or event a second Brogni that I had for another set of shields and HP Burn.  Any champion that brings survivability would work, even Steelskull.  Apothecary won't work as his A1 can randomly attack a spiderling, which in turn heals Agreth.

As long as you have enough healing and support the fight effectively becomes a matter of time.  Once I moved from trying to clear the spiderlings, it got quite trivial

Edit: Based on your roster, I think the following should work:

• Doompriest - Healing each turn plus debuff removal.
• Mausoleum Mage - Bulk remove debuffs and heal.  Also brings A1 slow for Agreth
• Sethallia - I wouldn't use her A2 personally.  Instead just use A1 to bring HP burn to Agreth and A3 to heal
• Scyl - Healing each turn and A1 slow.  Also a resurrect for emergancies
• Spare - Could be anyone that brings healing or support, or more damage if you are surviving well enough Fayne could be a good option for decrease attack, decrease defense, weaken and poison

Sethalia has to be booked/ maxed out to be effective, is that correct?

I realise there is no point sending bek or blind seer and my sethalia is not maxed out yet.

can anyone suggest a good team for agreth floor 90 normal for me based on champs that I have below:

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I could send Royal guard and play manual. he is maxed out

Bek is not good maxed out , I have trunda maxed out and mashalled .

I tried Armiger, Valerie, Reliquary tender , doompriest and scyl drake and failed.

Armiger is a bad choice in this fight. The Nether Spider will do a counterattack if the turnmeter is reduced, you don't want this effect here. Peydma would be an option, decreasing  the opponent's attack is allways good. Just disable her A2 (AoE attack).

Your account looks really strange. So many bad champs at lvl 50 and so many good ones, that deserve to be lvl 60, are only 50 or below.

which champs should i max out? i am happy to build a new team from scratch.

currently i have vizier maxed out and have a dupicate waiting in the wings. i have trunda, coldheart and blind seer maxed out, i also have sentinel and frozen banshee maxed out.

Are you looking to auto or just get past it?

You have plenty of champions that should allow you to finish normal floor 90 easily, assuming they have decent gear.  If I were you I'd just go the DPS route to have an AOE champ that kills the spiderlings each time they are summoned, and cleans the poisons.  Something like:

• Reliquary Tender.  A must for this fight.  Keep her at level 50, so will do her job, but ideally book her A2 to reduce cooldown to 3.  Only use her A2 whenever the spider uses his A2 to summon the spiderlings
• Scyl for healing each turn and resurrect.  Also positive afinity and brings slow for the boss
• Trunda or Big 'Un for the AOE nuke to get rid of spiderlings (they are magic affinity which is the positive one for this rotation Nether spider).  
• High Khatun for the speed aura and increased speed.  Just turn off her A3
• Spare slot for either more support or DPS, depending on how well you are faring.  More support would be Doompriest for her healing, or Hope for her shield.  DPS would be Royal Guard or Ma’Shalled

The key is timings.  Make sure you go into the boss with all of your abilities ready.  This means using just A1's when you finish up the second round.  Then on the first round make sure you target just the boss with single attack skills until either Reliquary Tender uses her cleanse or Trunda/Big 'Un uses their nuke to clear the spiderlings.

After that, make sure that Reliquary Tender's cleanse remains unused until the spiderlings are summoned again, and that the nuker's nuke is also ready

If you are still struggling then it might be a gear issue.  Can you show some of the stats?