You can try checking back the place where u save ostrava for the first time. There should be a flame there (if he is dead) containing the key and it should not disappear unless u take the key and drop it back.
Once I even got 2 keys in 1 playthrough. I killed him in boletaria, took the key and quickly tele back to nexus. Guess what, Ostrava is still sitting on the beam in nexus. I killed him again and its another key =.=.

As for armored spider, wear a thief ring and then start shooting arrow from the place u enter. If u dun wear thief ring, the boss will immediately shoot fireball and web at you. Still need to becareful though, after u shoot arrow the spider will blindly shoot fireball at u, so basically shoot 1 or 2 arrow then get ready to dodge. Just repeat the process till its dead. As for melee, I dun tink its a good idea cause the web near the boss can entangle u and it will stop u from rolling which means death.