Boss Fights

In terms of plot, I occasionally run into concerns on making any particular villain too powerful for their role, which crops up when trying to maintain a challenge with the party. I also worry about how to handle a villain that logically has the means to get himself raised, or some of the consequences if the villain actually had a life outside its lair. 

That's only a little bit of thought on plot considerations. Probably the most difficult aspect for me is the actual mechanical considerations. 

Past the low levels, a party of level appropriate characters can bring considerable force to bear without even using any teamwork. This makes 4v1 fights a challenge to design, as an 8th level party can easily dump over 100+ damage in a single round, making many lucky to survive two whole rounds. If action-denial or similar is used to much effect, then our boss is basically hoping for a good initiative to actually do anything. Finding anything that can survive and act for an entire three or four rounds is almost the same as finding something that can one-shot a character. 

The longer they last, the faster they can kill, which makes upgrading a boss difficult because a linear lifespan increase is an almost exponential damage output. 

Then there's the option of toning down the boss and giving him friends to help him, which can get cumbersome if ill-designed. You need to take into account that you need to plan well for this, because you can just as easily end up punishing yourself by making the boss not as big. As a rule, players are going to team up and focus fire on the boss. This is surprisingly easy with spells and bows (or flying melee if the minions aren't airborne), making the presence of minions superfluous in terms of actually extending the boss's lifespan. 

We return to the same problem. Putting a nametag on the villain makes their insurance premiums skyrocket, and life-expectancy is directly proportional to CPR (characters per round). I have contemplated using this in my games... 

Boss Template: Triple hit points, raise saving throws by +4. Increase CR by 2. 

It's a means of adding padding to the Rocket Launcher Tag that is D&D, theoretically allowing me to still have a boss that has an actual lifespan without as much worry about one-shotting anybody. However, it suffers from not really addressing the flaw of one-dimensional strategy, and those who don't enjoy throwing numerical combos don't have fun doing it an extra round or two.

One option that holds a lot more potential is giving the boss only medium offense but good defense, while carrying around a good size posse of minions that have low offense and VERY low defense. When you can one-shot the skeleton that does 1/3 the damage of the necromancer, then it's in the party's best tactical interests to save the necromancer for last if it takes so little as four attacks to take him down. However, a party is not usually that tactically minded and there exists A LOT of psychological traction to put on the blinders and focus fire with extreme prejudice, and this is assuming that the party can even see the offense/defense ratios between boss and underling.

Even then, with D&D in particular, it is common for character builds allow for extremes in damage output and not always in a very distributed fashion. The ones that do high damage do twice that of the low damage dealers, if we're lucky (I've seen a five-fold difference). Amongst those high damage dealers, some spread it out over three or more separate attacks while others dump it all into a focused all-or-nothing attack for the round. The survivability of the necromancer and his skeletons are going to vary wildly depending on which character's attacks are made as the baseline. If I use the lower value attack as the baseline, then the higher-damage character will have incentive to use his attacks efficiently on the necromancer, because the higher damage will be wasted on skeletons. If I make the necromancer require four attacks from the big hitter to go down, then I'm back to the original problem of finding such a creature that can survive four of those; the power-hitter of my 10th level game does ~120 per round, and I know of no such monster with 360+ HP that won't eat the party for breakfast.