So I went back to Sideways Cave at level 17 to get some revenge and finally clear out the last room with the gargoyles. Most of my kills were made using a single shot with the enchanted bow (+5 damage for each of the 3 damage types: fire / frost / shock). The regular imps die immediately due to the 3x sneak damage multiplier (10 pt arrows, 8 pt bow, 5 pts for each of the three enchants).

Only the mystical imps, trolls, or gargoyles required a bit more finesse. But because I have a sneaky mod installed (Attack and Hide), if I shoot from absolute darkness I can often get off 2-3 shots before they locate me.

The trick to the gargoyles is that you can't stay hidden. Otherwise after each time they take damage, they'll shield (reflect spell damage), fall over, and then regenerate. So you have to get their attention and let them shoot their drain spells at you. At which point, it's just a matter of shooting 5-6 arrows at them, each one with poison (I was using 4 pt over 17 seconds).

My last mystery is trying to figure out what the ayleid reliquary in the last room of Sideways Cave - Lost Abagarlas is for. It's a locked reliquary off to the right side in the caved in corridor in the last room.

(The answer is found on the OOO FAQ page. Basically if you have OOO 1.31 or later, you'll come across the key as part of the last mission of the Thieve's Guild.)