Goblins are frequent enemies in the early stages of a campaign. They have funny voices, they're distinct looking and they make for interesting combat and loot. But, time and time again goblins will prove to be formidable enemies even against higher level parties when used properly. This is not a guide on how to use goblins to properly roleplay goblins or use them in strange ways. This is how you can use goblins to kill your party.
But first, here's a free magic item stat block:
And now to business.
Ambush
The most critical part of properly utilizing goblin tactics is prep. Goblins use traps and hiding spots to give themselves a leg up on the enemy. Create traps that make noise and make it abundantly clear that the party has stumbled into something, while simultaneously alerting the goblins as to exactly where the party is. Because it's easy to make noise, these kinds of traps can be fashioned in pretty much any scenario.
Range
The action economy of the goblin allows it to either hide or disengage every single turn. As a DM, you should always attempt to use this bonus action. Keeping out of sight and out of easy melee range makes is priority number one. The hiding/disengaging allows for distance, and once there's distance the goblins can hide to prevent ranged attacks from landing properly. This makes it hard to actually do damage to a goblin without burning a spell slot, or moving, and as soon as party members start to move we begin with the third party of using goblins.
Multiple Attack Groups
Goblins are pretty low AC/HP enemies, which means you can assume at higher levels they're only going to be able to take one hit. If you bunch all your goblin friends in one area a single fireball or other form of AoE attack can wipe them all out with ease. To combat this, split your goblins up into small attack groups of three or four goblins per group. That way you can spread them out and don't have to stress as much about having the whole clan wiped out in a single round. Then, as the party splits up to chase down one attack group, another one which has been patiently waiting in hiding can begin attacking the most vulnerable member. Or, if the party does move as a group, other attack groups can consistently flank and pepper them with arrows while the initial group which instigated leads them toward more traps/goblins.
Success at Any Cost
Because goblins fall easily you can use a lot of them, which also makes spells that are other wise very strong, much weaker. Hold Monster and Command are very strong spells, but when cast on a single goblin to much less. This applies to goblin deaths as well. Playing the group of goblins as a group with victory as its goal allows you to sacrifice individual goblins in the name of the many. Have one lead players into an AoE trap, have another attack group act as a meat shield sucking up melee damage while the rest rain down fire from shortbows. If possible, a goblin will try to escape with its life, but it is part of a clan that must succeed at combats like this in order to survive.
Example Scenario
Here's one small example of an encounter that could prove difficult for any party by simply adjusting the numbers a little bit and damage on traps.
The goblins are split into four groups and have set up in a valley of some kind. Two of the groups are up the hillsides on the left and right, hiding. One group has fashioned a hiding place in the road itself, and the final group is down the road a ways, unhidden. As the party approaches the revealed group begins shooting arrows at them as soon as they can. Once the party is near the group hiding in the road, they emerge, attack, and then recover themselves using their bonus action. If the party pauses to deal with the concealed goblins, the goblins up the hills begin raining fire down. If the party instead begins to chase down the revealed party, the concealed goblins will wait until they're out of melee range before following, also raining down fire. There are traps set up farther down the road which the revealed group is leading the party towards. If the party does not continue deeper into the valley, the two groups on either mountain roll pre-placed boulders down towards them, sealing off the escape. If the the concealed party in the road is killed, one of the groups from the mountain rotates down so there is always a group directly behind the party. The traps set in the road are focused on preventing quick movement first, and damaging second. If any goblin in a group gets caught in some way the rest of the attack group will split in two with each new smaller group fleeing in opposite directions. Split parties will not group up with already existing parties, they will only retreat to a safe distance before once again resuming attacks on the party. The goblins will scatter if at some point it becomes clear they cannot kill the party.
So in short, use your action economy, deal damage at range, and keep the party constantly feeling like they are chasing the goblins without getting any closer. By making the party feel like there's no way to get away from the guerilla tactics of these pesky grey goofballs, the goblin will turn from a nuisance to a threat. And once they feel threatened, you can start throwing goblins at them all the time. It makes players with backstories revolving around hating goblins make a lot more sense.