My experience is that the best option for the 8th Chamber is either a cleric or a mage. Immediately after starting the 8th chamber, summon a Fire Elemental. Yes, it will go hostile. Don't worry about it. Run away and cast Invisibility (for a mage) or Sancturary (for a cleric) on yourself and your Fire Elemental will turn on the monks. And the great thing is that the FE will be immune to the fire blasts.
If you're playing in HOF mode, you can get even more nasty and cast a Gate spell. The demon will also be immune to fire and will tear the monks to shreds.
If you have trouble getting in your first summoning spell, perhaps your mage or cleric doesn't have high enough Concentration skill.
As other have said, you can try summoning something with a quicker casting time initially, before going Invis and casting your true nasty summons. In the 8th Chamber, the best summons are those who are REALLY nasty and immune to fire.
On a more general note, Clerics are very well suited to ALL 8 chambers, mostly due to their ability to summon all sorts of creatures, and the ability to cast Sancturary.
On the Spider chamber... This can be a very easy room or it can be a rather tough room, if you get hit, poisoned, and confused. My experience is that this room is great for a cleric or a mage. Clerics for the obvious summoning and sancturary reason. Mage, because they can cast some rather nasty other spells that work well on lowly spiders (Acid Fog, Sunfire, Symbol of Hopelessness, etc.), and Mirror Image is a useful defensive spell in this scenario.
This room can be a bit of a challenge in HOF mode, since the spiders won't die with a single fireball. You usually end up wanting to use a lot of summons to defeat this room in HOF.