My approach is:
Is that the mission where only one gnome is allowed, and you need to go down a hole and cannot retrieve items from your starting loot ?
Tentacle monsters are blue shielded, if I recall correctly. Therefore they are vulnerable to daze. And their attack mode is charm.
Therefore bring a gnome, nude. And arm her with a shotgun with daze ammo. Bring lots of extra ammo, a canteen, a pick-axe, and if available an item that reduce stun. I throw down the hole the loot items before venturing down in.
Good luck.