Since you wrote that your character is pretty good with a bow, I'll assume they have a high dexterity score, but a rather low strength one, right? In my experience, such a "dex build" can sometimes be a bit trickier to play, since they can't fall back on pure bashing power, should more magical or elegant means not be available. But they are playable in any case!
1) Unfortunately, I don't remember the exact skill levels needed to succesfully open that door, but I think it was something in the ~20 area. Unless you plan on maxing one of those skills anyway, I don't think it's worth sinking all those skill points into it just for this door.
2) You should be able to retreat into the tunnel you came from once you triggered the bebilith's attack and be out of it's reach. Using your bow to wear it down will take a while, though.
Alternatively, you could try to run past it (using "hide in plain sight" if necessary) and get to the obelisk this way. Activate it and deal with the Bebilith afterwards. Should be way easier having once again access to some magical gear.
Another option would be to bring Valen along, if only for this area. He should be able to deal with those spiders and probably even the bebilith. You can support him from a distance using your bow and sneak attacks.
