First of all, remember that the purpose of the game is for players (and, secondarily, the GM) to have fun. As such, ask yourself if your battles need to be more difficult. Do you feel the ease in which the players beat all your monsters makes them bored, or are they excited that their tactics are so successful?
Remember that the game is not you versus the players. You're all playing together, and the purpose of the game is to have fun. You don't have to beat them.
Okay, next thought.
I agree that, in some ways, conventional tactics can be useful to GMs. However, those tactics were devised for situations where the participants were in the thrall of nature. As a GM, you define it. Let me explain what it means. You have essentially many ways to provide (combat) challenges for your players, some of which are:
- Powerful enemies
- Number of enemies
- Complex multi-enemy scenarios. Tactics become a much more difficult problem when you're facing multiple enemies of different kinds, of differing power levels, some of which are hostile to one another.
- Environmental obstacles, such as:
- Motion-inhibiting (difficult terrain, barriers, lava lakes)
- Perception-inhibiting (fog, darkness, noise)
- Ability-inhibiting (underwater, anti-magic, EMP)
- Damaging (dangerous plants, noxious gas, spikes)
- Asymmetry between the effect of environmental conditions (provide an advantage for enemies, and disadvantage for PCs)
- You always know who your players are. Their strengths, weaknesses, etc. Design your encounters with that in mind.
- You can always surprise them, literally or figuratively.
- Red-herrings. Make your players expect something, and pull something else.
- Betrayal, deceit, etc, by the characters you control.
- Misrepresent the combat situation. Make enemies more intimidating than they seem.
- Scenarios don't have to remain static. Conjure reinforcements from thin air, adapt the abilities of enemies to suit your players' tactics. Give quiet bonuses. Fudge die rolls.
Note that the mere existence of some of those things (without any long-lasting effort on your part) can have a profound effect on battle, and possibly, on your players' lust for life.
Okay, there's some more, but this is what I have off the top of my head.
The point is, don't restrict yourself to mundane tactics. Use your omnipotence to sculpt scenarios in which you can crush your players, body and mind. Shatter their psyche, and instill in them deep-seated fears of multi-sided dice.
Edit: Hmm, this looks like an interesting project. I think I'll make a more detailed and organized list of ways in which you can make players' lives a living hell. What was that? 'The purpose of the game is to have fun', you say? Why, whoever could have told you such nonsense?