This is going to be a fun ride.
For the purposes of this question, we'll assume this slime has similar qualities to the kind you find on social media (the one 7-year-olds are really into).
Make it really big
There's basically no limit to how big this slime can get, provided that it can receive food. You could have one enormous slime blob, which will basically be impossible to kill (barring some magic kill-spell). Even by conventional means, without advanced weaponry, this slime would pose a huge threat to travelers.
You could have a thin layer of slime coat every surface, which can blob together into one huge slime when provoked, provided that every unit of the slime is uniform, and that it can be cut into parts without damaging it.
This gets even more exciting, as every single unit of slime can act on its own as well, meaning that the slime could split up into thousands of virtually indestructible droplets, which can crawl through the nooks and crannies of our hero-soon-to-be-dead's armor.
Side point: Our slime is already basically invincible.
The semi-liquid qualities of slime mean that slicing or shooting it will do absolutely nothing; physical attacks are useless. Fire won't work either; the slime isn't flammable. The only way to really defeat the slime is magic, a lot of fire, or nuclear weapons.
Of course, the slime may be unkillable, but is that true for our hero as well? This brings us to our next point..
One of the most obvious solutions is poison. There are a lot of organisms in the real world (some frogs for instance) that can definitely kill a person or two easily. You can make your slime synthesize similar compounds, and suddenly that nondescript puddle becomes extremely dangerous to travelers in the forest.
You don't even need to kill the predator. Once you incapacitate them, you can suffocate them with the sheer volume of the slime (see point 1).
This builds off of the poison point especially, but you can have a massive slime disguised by a layer of leaves, etc. on top of it, meaning that there's virtually no way to determine what is a slime and what isn't.
The slime is basically a liquid at will, which means it can kinda go anywhere. Through gutters, pipes, cracks in walls, you name it. Nowhere is safe; if it's enclosed, the slime can just surround it and suffocate you. Slime in an enclosed space is simply horrifying, since it can just go around you and slowly close in on you.
You know what they say: When you're being consumed by a slime, no one can hear you scream.
Putting it all together
Bob the Adventurer is taking a walk in the forest. All of a sudden, he steps into a little mud puddle. No biggie, he can clean it off..
Then the slime jumps into actions. Thousands of tiny slimes spray off of the subterranean mass, crawling through Bob's armor and onto his exposed skin. He doesn't even have time to scream before the slime encapsulates him. He is slowly pulled into the underground lair of the slime hive-mind, never to see the light of day again.
Another day in the forest goes on.
Making It Useful
There's plenty of uses for the slime, given that you don't provoke its voracious appetite. For instance, need to get rid of basically any organic matter? Just toss it into the slime. Your incredibly dangerous super-slime could solve our world's trash worries, and there are doubtless other ways to exploit the slime's behavior I haven't thought of.
Another Distraction - Realizing the Slime
Your slime could use photosynthesis to get energy (remember that it can spread itself over a veeeery large area) in addition to whatever prey it can get. Effectively, it's just a huge glue trap that's also sentient and can move..
Reproducing is no problem for the slime. There's no concept of individual slimes; they don't operate discretely. Rather, they can be temporarily split, but normally operate as one hive-mind, serving its own needs.
Making the slime as smart as it is may be a little problematic, but a little handwaving may suffice. The only way for the slime to transmit signals rapidly would be chemical signaling (which would be slow for the massive slime), but that's a detail the readers don't need to worry about.