There is a new anime that's been getting talked about a lot. It's called Goblin Slayer. You can probably find the episodes on Youtube, though I'll warn you before you do that the first episode is very dark. By dark I mean brutality, death, nudity in a way I can only describe as violent (and that is supposed to make you uncomfortable), and, yes, rape. The rape is not directly on screen, but it's not far from it. If any of this is going to bother you, do not watch the first episode of Goblin Slayer. The later episodes are not as dark - since the world is already established - but these are key parts of the world and they're front and center in all their brutality in episode one.
That said, if you can stomach those things, Goblin Slayer is a good show that in a lot of ways mimics many a D&D game. Adventurers go out on jobs for coin returns where they solve problems. Higher ranked adventurers fight bigger monsters like demons, dragons, ogres, etc. Lower ranked adventurers do less rewarding jobs which often means things like goblins. Just, in this world, the Goblins very much have a Tucker's Kobolds vibe going for them. Today I want to talk about that.
Where You Fight The Monster Makes All The Difference
One of the points made early on in Goblin Slayer - in Episode 2 I believe - is that there is a big difference between fighting goblins in a town they're raiding, and fighting them in their home. Many people face off against these goblins in a town and chase them off with relative ease. Why wouldn't they? A lone goblin, while vicious, is relatively weak. They're cowardly. You kill a couple, and the others are likely to scatter and run, then boom, you've managed to beat the goblins, easy peasy, right?
However, at their home a couple things change. One, they probably have the advantage of numbers, not you. Two, they have the home field advantage, and that can pay off in a number of ways.
Sacrificial Tactics Can Take Down A Bigger Enemy
One of the things you see early on in Goblin Slayer is the tactics the goblins use. Goblins, despite being cowardly, are also vicious. In a fight - especially one to defend their home - one or two will happily throw themselves onto an opponent to make an opening. In D&D terms, this boils down to using a numbers advantage to do more than just attack a PC.
I mean, why should 6 opponents all throw their one attack at a PC, when two of them can grapple and restrain the PC giving the other 4 bonuses on their attacks, or otherwise out-right making the character unable to defend themselves as they quickly go from grappled to restrained to pinned and then, once they're held and helpless a lone one can go for the finishing blow.
Dirty Tricks Are Welcome
I remember in the AD&D 2nd Ed days that goblins were always a nasty opponent to fight. This wasn't because they were super strong, but rather because their weapons were always considered poisoned. Goblins liberally coated their weapons in excrement and other things. Every blow from the weapon was accompanied with a saving throw or you ended up diseased, poisoned, or with some other condition.
The same can be done with goblins, and low level mobs, in any game. Poisoned blades, arrows, traps, and abut everything else. This goes along with sacrificial tactics, but status effects can do a lot to make an encounter that much harder or costly, which in turn can do a lot to even the scales in an encounter.
The Smaller The Creature, The Smaller The Home
You know what's weird to me? In so many games I see small creatures have nests that are perfectly comfortable for creatures a full size category bigger to fight in. Now, obviously, this is partly done to make the adventure completable by the PCs, and there does need to be room for medium creatures to move through a map if you want them to complete it. Still, a goblin or kobold warren - creatures that tend to be 3' tall at max and fight with thrusting weapons like daggers and spears - shouldn't live in a place where someone can go to town swinging a 3-6' blade around, right?
Beware The Consequences of Doing This
As a final note, you want to beware the consequence of employing all these things. "Imagination is a weapon. Those who don't use it die first" is a quote from Goblin Slayer, and it is very appropriate for RPGs. If you go and use tactics and planning to make 'simpler' monsters more dangerous, expect your players to respond in kind. This isn't a bad thing. You could even end up with a game that is a lot more fun for you both to play - even if it does mean some more work. However, once you get your players using their imaginations to solve problems, they're not going to stop.
GMs looking to challenge people like this are where you get the PCs from the truly absurd stories. It's where you get PCs who will buy an entire town's oil supply and a drill to fill a desecrated temple with oil before throwing a match in to kill all the monsters inside. It's where rather than risk going into a cave system full of goblins, the PCs will just plug up the air holes and use fire to suck all the air out.
There's nothing wrong with this. You just need to be ready for it to happen, and if it doesn't happen, you're going to kill a lot of PCs.