Here are two more points to follow up on yesterday's Best Practices of DF Monster Stocking.
More Mixed Monster Encounters
Make sure you combine monsters with each other.
Trolls and rust monsters are natural allies. Trolls and stirges are natural allies. Demon-apes and gargoyles are natural allies. Shambling mounds and druids with the Lightning spell are natural allies. All at least according to my players.
Why is that? They've encountered them together (or had one pile in to a fight with the other) or thought about how nasty they'd be as an encounter together. Sometimes it's a strange symbiosis - trolls don't have weapons, so they don't mind feeding delver armor bits to their rust monster. Sometimes it's an obvious pairing (druids can heal plant-men). But having two, three, or more types of monsters together potentially makes the threat fundamentally worse.
One monster may have an exploitable weakness, but their fellows do not. For example, a mix of flying and non-flying monsters means flying ranged weapon using delvers can't just fly up and shoot them to death. Or a monster might be especially vulnerable to magic but is coupled with a monster that can only be effectively fought with magic - do you focus on taking out the ones with the weakness or use your magic against the otherwise invulnerable enemy? The class wizard-backing-fighters approach is exactly this - each aids the other and covers a weakness.
A mix that doesn't enhance one or the other is just different in color, not kind. An ogre with some orcs is a nice combo, but it's just a physical big foe with physical strong foes. Sufficient sword skill will see them off. A wizard with three apprentices - or just four different wizards - are just a magical threat. Sufficient Magic Resistance, Will, and HT rolls will shrug off their non-physical attacks and if you can dodge a wrench you can dodge a Fireball. You might need Resist Fire and Resist Lightning and Resist Acid to fight them, which is more challenging, but it's just like the ogre and the orcs. One might be stronger but the threat is identical.
Add a wizard to the orcs, or have them summon an illusion-casting demon and you've got a very different encounter. Give those four wizards a few excellent melee fighters or draugr bodyguards and you'll need to choose between Resist spells and other buffs. Do you bum rush the wizard and give up your flank and your squishies, or do you play like-fights-like?
And don't confuse the origin and color description of an attack with its effect. An nearly-unkillable demon doing 4d+4 with its mystical demonic thighbone club and an ogre doing 4d+4 crushing with a club are the same offensive threat. A jet of flame that can be blocked with a shield is not that different from an arrow that can be blocked with a shield. A paralytic touch attack resisted with HT and a paralytic contact poison delivered by touch and is resisted with HT aren't terribly different, either - both are easily handled with the same Dodge score. Understand the game effect of the color and what it'll play like when mixing threats. A mix of ghouls, wights, and deathtouch wizards is all one threat with different damage types - it's keep away. The same mix with Curse-spell casting evil clerics might be a wholly different encounter.
Players, keep all of that in mind. Each fight isn't "find the tactical puzzle solution." A single monster might be that - hey, they're good at melee, let's not do that. A multiple monster fight will force you to prioritize. Make sure you actually do that on some level. But don't miss the forest (winning the fight) for the trees (finding the best tactic to neutralize the combination).
Telegraph the enemy
Leave clues. As a GM, it's tempting to make all monster encounters a surprise. But especially in GURPS DF, the templates come with serious investments in knowledge skills. Physiology to spot vital points. Hidden Lore to know secrets about types of creatures. Current Events and Research to gather knowledge in town. Naturalist to understand animal types. And so on. If you can't use them except when the monster appears out of nowhere to attack, it feels like they have less value.
Think about what those creatures would do to their environment. Do trolls sharpen their claws on walls? Do eyes of death shoot down every damn squirrel in the area (yes!)? Do peshkali have a snaky musk? Do giant apes leave dead snakes and dinosaurs lying around?
Color like that works in several ways. First, it's more immersive in the game world. Only in Munchkin, Dungeon!, and video games do monsters just sit in rooms and get generated by a random element when you kick the doors down. Second, it gives the players a reason to use their skills on their sheets. Third, it gives the players to use their own brains and attention to in-game details ("What, peshkali? WTF?" "Dude, didn't you hear him say 'a snaky musk' and mention the sounds of swords being sharpened?") Finally, it encounters scouting and looking and sneaking, because that's how you get these before the first sign of horde pygmies is, "Oops, dart in your neck!"
Players, this means you're rewarded for looking for clues. Oh, and listening while the GM is talking. Sure, that was a good parody video you saw yesterday and it does remind everyone of that time with the orcs and the barrel of alchemist's fire . . . but what's going on around you now? Scout, look, listen, use your own knowledge to spot incoming threats . . . then use your character's skills, basic testing, and your own experience to confirm them. If you don't pay attention to clues, the GM won't bother to put them in. If you look and the GM didn't bother, that's a hint that you want them!