So I've DM'd three separate games, of which one that I'm largely happy with. However, one consistent problem I run into is designing fun, exciting, and interesting boss fights. I've run into two major problems.
The Blitz
The boss fights are over too quickly. Generally this boils down to a problem of the players winning initiative overwhelmingly, and bum rushing the boss. This is an older problem, and one I've been able to largely solve, but I've run into it once or twice since my first mistake with it. I've added minions, generally my boss enemies have Improved Initiative. Still, it happens when I underestimate my party's damage-per-round.
The Slog
This problem is more commonly occurring. Fights turn into slogs. The boss doesn't get hit (usually from either a high AC, a lot of HP, or some other defensive mechanic like miss chance or some such). This is even worse than the Blitz, because instead of saying "that was easy," it boils down to "that was boring and a waste of my time." This can also happen from status effects like Fear, Hold Person, and other similar Save-or-Suck effects. But I don't know how to make a boss intimidating (and ergo, exciting), without using the strong Save-or-Suck effects.
For example, just recently I had my gaming group fight an evil cleric, in which I planned using minions, but thanks to unforeseen circumstance, the minions were quickly eliminated by the party's cleric. I planned on using them to harass the players while the cleric boosted the minions, so he had a lot of status effects. Unfortunately, when those minions were eliminated too quickly with Channel Energy, and the strategy was shot. He had a Deeper Darkness spell as a last resort, but it ended up bringing the fight to even more of a slog.
So my question here is twofold:
What specifically am I doing wrong, and what can I do to fix it?
Best Answer
The biggest key to creating interesting boss fights (in my experience) is to introduce an element of surprise or guess work. Fights are boring if they're just constant dice rolls back and forth where everything goes as expected. But you can make mechanics which keep the players guessing and on their toes, which force them to constantly be thinking about what's going on or what to do next.
For example, in the culmination to one of my campaigns, I had the final boss have a "shifting immunity". Each round, I would roll on a table to see what type of energy or effect he was immune to that round, and each immunity had an associated color. So each round, the boss would change colors and have different things it was vulnerable to.
This forced the players to be paying attention and didn't allow them to just sit back and spam the same moves over and over. ("What? That fireball worked last time...") It also made it interesting for them, because, in addition to being a shifting immunity, it would grant a different power to the boss.
That's the basic premise. If you want a fight to be long, keep it interesting with different mechanics. This can be something like I've described above, or a "phased" fight (i.e., the boss has several different stages or modes), or something like a very specific effect he/she/it is vulnerable to for a killing blow. It forces you to get creative, but there's no way around that; if you just rely on what's in the source books, well... the players have read those just like you have.