I've beefed up foes with bigger stats and it it works. But the thing I have found makes the monsters tougher, is making "harder moves" across the board for the deadlier monsters like Dragons. You have to get out of the blow by blow/round by round mentality of combat like so many other rpgs.
GM "Your fighter hack and slashes the dragon, and (gets a 9) success with a cost. You deal your damage and your greatsword slides between a scale and imbeds deep, in fact you are having a hard time removing it.. the dragon howls in pain, he lifts into the air..what do you do? Hold on or let him take your ancestors enchanted" weapon?
"Uh...oh boy...I'm holding on..?"
GM "Ok, he flies high into the air, a few hundred feet and notices you, he starts shaking and making violent moves to throw you off! What do you do?"
"I'm holding on and trying to push the sword in deeper so it doesn't come out"
GM- "OK, defy danger using Strength"
ROLLS a 6....failure...
Now here I could say, the dragon was swooping down as the fighter falls...and maybe make up some stuff about plunging through some thatched roofs which helps break the fall..and then still deal a hefty amount of damage...
Or..maybe he doesn't fall..maybe he does plunge it deeper...further enraged, the dragon swoops down burning half the town down killing many including somebody important (which will change the plot so far, completely), or smashing/landing on another character , or maybe picking that character up in it's claws and flying off with both of them.
DW is like making an improvised action movie, and that takes getting used to.