I think a good way to beat the high Spider levels is the hp-burn strategy. Put hp-burn on all small Spiderlings, do not kill them, keep the turnmeter of the big spider down to prevent her healing and wait.

You have Drexthar and can try to use him on Spider 19, probably you want to beat that stage only once: Drexthar, Armiger, Armiger, Vrask (to heal your team), Maus Mage (cleanse poison, heal) or something like that.

In stage 20 you can use the same strategy, the small Spiders will probably attack Drexthar for his weak affinity even without provoke. Just make sure he has enough hp and def to survive that until all of them have their hp-burn.

Another, more classic strategy for 20: ignore hp burn and just wipe her out with your two Royal Guards. Seeker for Spd buff + atk buff, Stag Knight (or anybody else, Tayrel, Draco) for def down, 2 Royal Guards. The base damage of Royal Guard's A2 is 2 atk + 10% of the enemy max hp. The enemy def decreases that damage, critdamage can incease it. If your Royal Guards can crit very hard, you can deal an enormous ammount of damage to the big spider.

Can'tKeep Drexar Alive 

Wich stage, 19 or 20? And what kills him, the hit damage or the poison?

If it's the poison, change the turnorder of your team, so that Maus Mage has his turn before Drexthar and can remove the poisons.

The hit and 19

Done it once which was hard And 20 is easy, thanks a 

19  was  hard  for  me  as  well.  Thankfully  my  team  stomped  20.