Invisibility is a great tool for bad guys who need a few more moments to get into position, summon some more monsters and so forth. It's used so much in material that my players always always buff with See Invisibility. Can't blame them.
But that's making encounters rather predictable. What are some other easily-accessible tricks bad guys could be using to get just a bit more room and time to do stuff?
My five man group of 13th level characters had a rough time of it when they encountered four 10th level druids in earth elemental form. They popped out of the cave walls and dropped transmute rock to mud followed by transmute mud to rock on them.
Similarly, they near TPKed later when facing four 10th level slayers with archery builds, fog-cutting lenses and some smokesticks.
Basically it comes down to terrain and vision. If the bad guys have a way to move through somewhere the party doesn't, or if they can see the party when the players can't see them, they'll have a significant advantage. Environmental hazards combined with creatures that have immunity to those hazards are a classic, like undead in a cloudkill, or a fire elemental in a resetting burning hands trap. Illusions that enemies see through but players don't are also good options.
- Darkness / Deeper Darkness, although that should not be overused.
- Cannon-fodder minions, just to keep them away for a while.
- Misguided minions/hostages, that changes the dynamic of the combat. Especially if the BBEG is willing to hurt them as well (Restrict their movement while I cast Empowered, Maximized Fireball)
- Fog / Smoke tactics. Obscuring mist is something that comes online quick.
- Minions with crowd control options. The various versions of Entangle (vanilla, burning, sickening, thorny), Pit spells, the aforementioned fog/smoke tactics, Stone Call, Vomit Swarm, Burning Sands, etc, etc.
- Outsource the buffing/summoning. The BBEG is a beatstick that has several caster minions capable of buffing him.
- BBEG is a Hangover Cleric build (Variant Channeler focused). Quick Channel a serious debuff (something like Envy or Madness is funny), use standard to buff or summon with Sacred Summons (if access to useful creatures for that feat).
- Synergistic enemies. I know of the classic Shocker Lizards with a Shambling Mound, the Mummies covered with Brown Mold, Gnolls carrying Pugwampies around, Undead in a Hungry Fog.
- Anything that requires the players immediate attention. The fire-setting that avr mentioned is a good example. Or another enemy that is currently holding onto a McGuffin that has decided to bail.
- Enemies with Terrain Advantage, either something simple as flyers over swampy terrain, or higher ground for ranged attackers.
- Mobile enemies (Spring Attack) and someone else who casts a jacked up Slow (I believe Fey creatures usually have such options,
like Quicklings and small Fey with Bard levels).
- Invisibility doesn't work, but there is still Stealth, or even Disguise to move around before combat starts.
- Hide behind Illusions (wall or something like that) and cast spells without Vocal Components (either Still Spell, or Psychic Casters)
There are a couple of things that you can do. If the players do not have extremely high perceptions rolls stealth can be useful. Even if they have high perception rolls it is probably easier to get a high stealth roll than perception. Things like elven cloaks and shadow armor work well.
Illusions can also work well here. Illusion of calm is perfect for this. Mirror Image will also give some time to prep. Displacement gives a 50% miss chance which can allow you to get off a spell or other preparation. Illusionary wall will prevent them from being seen. Project Image not only hides your location but allows you to cast spells so they originate from the duplicate instead of you.
Another good trick is to disguise a minion as the BBEG and have him cast some sort of spell. Chances are all the players are going to go after the minion instead of the BBEG. Some if not all the players are going to blow their action on taking down the decoy leaving the BBEG a free round to act. If you really want to be evil have the decoy be an innocent the BBEG tricked or charmed into cooperating.