Pack tactics.
You don't really have to add the NPC or PC class levels to them (which would, by the way, affect the CR quite significantly, depending on number of "leveled" critters you use). All you have to do is "think like a pack of goblins". Here are several tactics that my goblins commonly use:
Hit and Run: They will show up, let loose a volley of arrows, bolts or whatever other projectiles they have, then run away.
Cannon Fodder: Groups of goblins will charge in, assume either defensive posture or even go full defense and delay / corner PCs while other Goblins pepper them with ranged attacks, reach attacks with polearms, alchemists fires etc.
Surgical Strike: Goblin gangs will team up to swarm what look like most vulnerable (non-armored, lightly armed) party members, jointly tackle, grapple and bring them down, then stick 'em full of daggerholes.
Traps, Traps Everywhere: If goblins are playing on the home field, they're bound to use the home field advantage. From installing every possible type of trap they can think of to using pre-existing terrain features such as dense overgrowth, low ceilings, slippery or difficult terrain etc, if it's an option, they'll use it.
...and then you start combining the above. They will taunt and provoke your party to pursue them - through traps and movement-hindering terrain, all the while hitting the party's flanks with volleys of ranged attacks. Then they'll use the cannon fodder to ensure that PCs stay in one place until an inricate, "takes time to go off" trap such as landslide or a cage of rabid beasts goes off. Then, while your fighters are busy dealing with those rabid wombats, they'll sneak up on the party's back and quickly take down the wizard.
If you want to get worse, go with above and start adding in PC class levels; in combination with above, even a 1st level caster can wreak havoc with a couple of Greases, Obscuring Mists or illusions; an alchemist will make party wish GM was dead and boy, wait 'till you see their faces when they get hit by a couple of raging goblin barbarians.
A well-planned and executed attack by a large group of low-CR intelligent monsters could easily be one of biggest nightmares for any group, regardless of CR. Sure, there'll be no massive blows that leave party's AC 25, 75 HP fighter dead in the dust, but the engagement will whittle away at the party, forcing them to waste spells and other resources, bleeding them dry very gradually.