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[holy cherrys]

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[sonic_fanrs]

Here's a quote from the Runescape Wiki:

This spider is more often found in Large Dungeons rather than in small or medium dungeons, but this is only because there are more rooms for the spider to spawn in. Furthermore, the chance of it appearing is also influenced by the Slayer level(s) of the party member(s). Therefore, players or teams with higher Slayer levels are more likely to encounter the spider. The spiders are more likely to appear on Abandoned II (30-35) floors, so hood seekers are advised to use these floors when they attempt to get one, but at your own risk, as there are usually all three types of combat in its room.

You should probably read the article; there seems to be a lot of ifs-and-buts:
http://runescape.wikia.com/wiki/Night_spider

 

[TurmMeOn]

just do dungeons as normal and check every dead end, there is no method i'm afraid.

 

[youtube]rYX4x_pm5Eg[/youtube]

I'm really not sure if it works though. I got mine when I purchased some dungeoneering training.

 

[Zerkerfist]

I haven't dungeoneered in awhile, but I find SSH's all the time, and it is ALWAYS while doing a large 5:5 on floors 30+

 

[ignitiion]

Just do 5:5 floors 30+! Normally I run into at least 1-2 spiders per dungeon. Lots of people already have SSH too, so if they get one, just ask nicely for it! You should get one eventually, don't dwell on it, as there is no real way to get it faster!

^Zerkerfist beat me to it by a minute.