This is a system-agnostic recommendation:
Play your bad guys smart!
I want to direct you to what I consider the bible on the topic, John Wick's Play Dirty. But here are some examples of what I mean:
Remember that most of the time, PCs are invading a territory the monsters / bad guys / enemies know like the back of their hands. A dungeon is the NPCs home. They'll use every advantage:
- Choke points
- Escape routes
- Blockable passages
Plus, like I said, the bad guys live there. They have had all the time in the world before these arrogant adventurers showed up to turn their home into a meat grinder for intruders. They'll do all of this and more:
- Use and even create cover to fire from protected positions
- Spy on the intruders and report in time to make intelligent decisions
- They'll lay traps that would make you scream if you were a player
Don't forget that as well as living in the adventure environment, the bad guys also live in the game world. They know the rules as well as the PCs, and they'll use them to their advantage.
In Savage Worlds, this means they'll scare, taunt, and disorient PCs to score Shaken results. Let's say you've got a typical fantasy dungeon populated with a whole tribe of small, evil, non-humans - like goblins. I never played a fantasy game in SW, but I know that the TN for ranged combat is just 4+2 per range increment - take 100 goblins, firing in waves of 50 each (so half are firing while half are nocking another arrow). Somebody is gonna get hit! Even if they're only Shaken - that's when the cadre of knife-wielding goblins rushes out - each one intent on killing a human so he can be a candidate to marry the chief's daughter!
You don't need bigger, badder, monsters to challenge badass PCs. You just need bad guys that treat encounters like life-and-death situations for themselves, their families, and / or their cultures.
Play smart. Play tough. Play, as John says, dirty.
EDIT: Tucker's Kobolds are another perfect example of what I'm talking about here.