Here is a third post in the series I started a couple days back.
Just one best practice today:
Monsters, not Monster
I said this back when my blog was young - one-on-many is death for the one in GURPS.
Especially in DF, think multiple monsters. You can make a single opponent a fight against a group, but it's got to be an overwhelmingly powerful individual. Even then, being one big target isn't very good for the one big target. Two ways to avoid this with a monster is to make it monsters, plural.
You can do this by adding lesser minions. You can do this by including multiples of the same monster. Don't feel restricted by the "usual" number of a type of monster. Doppelgangers usually operate solo, there is usually only one dragon, there is only one tarrasque. Okay, but not this time. Even a second monster means the delvers have to decide how to split their attacks and how to defend to avoid getting double-teamed.
Players, not much I can tell you here. Don't assume monsters that tend to come singly in fiction, D&D, or video games is going to come solo in GURPS DF. It's always worth having a high-Per hireling or PC watching your six, just in case there is two of something. Keeping a tactical reserve isn't a bad idea, either.