I've added
Your team is able to beat the Spider. It's not a problem of champs, but of their level (Armiger and Tayrel are only lvl 50) and maybe of their gear.
UDK needs tons of hp and def, he will take all the damage. And, for this particular dungeon, he should be slow. Nekhret can put a block debuffs and ally protection on him for 3 turns, but the turn countdown is on UDK, not on Nekhret. If UDK is faster than Nekhret, he would stand without buffs after 3 turns. Maybe that's the problem. Or your Armiger and Tayrel are to slow and can't hold down the Spider's turnmeter.