A couple of thoughts...
As several have said...going without specifics makes it a bit tough. However, let me start by asking several questions:
(1) Where do the PCs fit in your campaign's food chain? At their current level...are they a significant force whose reputation is spread by bards far and wide or do archmages run lemonade stands on the corners of your hamlets? The latter is not meant derisively, just trying to get a feel for levels.
(2) Is the campaign a series of unrelated adventures, loosely connected scenarios or does it have a central over-riding, well-connected theme (or several main themes)? Have the PCs run afoul of the same group and/or groups repeatedly or is each new villain an island onto him/her/itself?
Assuming there is some continuity in the campaign from the stand point of plot and oppenents, consider the following:
(1) Intelligent opponents
WILL study the PCs. IMO, any villain with a 16+ intelligence who has either directly or indirectly faced the PCs before or who knows about them due to there sterling reputation or black infamy is going to do some "opposition research". Spies, scrying, buying drinks at the pub for friends, family and acquaintances of the PCs, bribery and blackmail are all methods to get info about how the PCs operate, what obvious spells/magic items/tactics the PCs have used to win their battles and what, if any, weaknesses they have evidinced.
Heck, the PCs themselves could divulge such information to that lovely young bard at the tavern as she gushes, "Oh please tell me how you defeated Foozle the Magnificent...I just love that story!"
If push comes to shove, the villain could even use
Speak with Dead or other magics to interview the PCs erstwhile foes and current wormfood to find out what killed them and how.
The bottom line is...if the PCs are well-known and if they use the same standard operating procedure again and again and again without taking precautions or varying their routine, then even a moderately intelligent opponent who has done some research should be able to hand them their collective a$$es. A smart opponent might even send a couple of different types of catapult fodder against them and scry the ensuing encounter to catalog spells, defenses and after-action actions taken by the PCs.
(2) As someone above mentioned, maybe the players are better at tactics than you are...which may mean you need to brush up on tactics or throw some new ones into the mix. First, are you using the opponents skills/powers/intelligence to the best effect?
Most creatures, unless very stupid or supremely confident, will not fight to the death...yet, in my experience, that is how many DMs play things. Creatures fight to the last, even when the situation looks hopeless...but he who runs away lives to fight another day

! Tactics such as feigned retreat, playing dead, luring onto more favorable defensive ground, etc get too little play. If caught by suprise and in an unfavorable situation, well-trained critters should break, fleeing in 10 different directions and re-assemble at a rally point to launch an effective counter attack instead of sitting around waiting to get stacked up like so much cordwood.
Creatures attacked in their own lair should, unless very chaotic or completely idiotic, have contigency plans and pre-planned reactions to assaults. If the PCs slaughtered a bunch of slaad and the slaad have reason to believe they are still in the complex, then it is very reasonable to assume that they would conduct a thorough sweep - within the confines of their abilities - to root out and kill the intruders. Scrying the PCs is one way to go, as would various clerical divination spells (if available) or a
locate object spell for an obvious piece of loot or treasure the PCs took with them after the first encounter. Your PCs might think it is cheese...but if they rope-tricked to rest/refit while still inside the complex, then I would say there is a very good chance the slaad would be waiting for them...and might hit them right after the first couple of PCs slither down the rope.
(3) Sunders, trips, disarms, over-runs, bull-rush, grappling, aid other, concentration of fire and ganging up on the weak link are your friends...especially nice things like bull-rushing, grabbing or grappling into enviornments the critter is protected against or resistant to, but the PCs aren't ready for. An example that springs to mind here is a giant, or other big strong critter, with stoneskin and other protections in place that imbibes a potion of water breathing, busts through the front-line tanks, grapples the party wizard and jumps off a cliff into deep lake. No matter how many hit points you have, once you start drowning...ya only got three rounds.
(4) You didn't mention alignments...but if the PCs are primarily good (and alignment has teeth in your game) then moral dilemmas that force the PCs to fight with one hand tied behind their collective backs is always fun. Dominated townsfolk, hostage interspersed with the bad guys or implications from killing certain foes are always fun. Some players may take the tact of "well...we can always raise them later."
But what if Joe Peasent doesn't want to be raised? The after-life is a whole lot better than his miserable existence, but now his wife and 12 children will go hungry. The widow sues him in court and the PCs are forced to pay support. If their actions kill a lot of innocents...they may even have a class action lawsuit brought against them...forcing forfiture of gold and goodies

!
~ Old One