Ok, this is the absolute best strategy: it's safe and slow, but it gets the job done better than hacking and slashing, I'll list each of it's attacks (and the leads for those attacks), then what you should be doing.

You start at the end of a tunnel, you need to run toward the spider. At this point, it can use two attacks on you:

-It gets up on its haunches and shoots web at you from it's rear

You should be prepared to roll to the side so the webbing doesn't hit you. If it does, it doesn't do much damage, but it will slow you tremendously, which could be more dangerous. Also, if you are slowed, do not use roll to dodge attacks, as most attacks will now hit you mid animation and do full damage. Instead, just block until it wears off

- It shoots volleys of fireballs at you.

Keep your shield up and attempt to roll out of the way of the fireballs. Likely, you won't be able to dodge some of these, so if you have your shield up it'll minimize damage taken.

Continued in "reply" section below