Okay, so my answer will be based on my own experience.
To kill Tremor Worm you have to go on -2 level of Glooth Factory to the west, during Wildlife raid and stand here. This place is the best not because it's near to the entrance, but there is almost none of monsters even during raids so you're quite safe even on lower level. Now you have to stand here and wait for the boss, because it appears for a moment and dissapears in a few places around.
You can kill this boss even solo. I killed Tremor Worm on level 400 (Elder Druid) without any summon, using only waves and SDs. The problem is that it took me around 2 hours. So the conclusion is that the more people, the better. Vocations and levels are not so important.