High level combat perplexes and confuses countless GMs. Players are literally running around with characters who can jump canyons, alter reality, and more or less live forever. They are as close to god-hood as you can get in D&D without a divine rank; and in real-life terms they are already there.
This is very confusing when you're trying to GM a high level game. You often have little idea what to expect; and even more often, you believe you have crafted the ultimate villain, ready to stand strong against the legions of PCs out there, only for them to be hosed in .5 rounds by a quickened spell that would make the Death-star jelous of the PC's power.
The biggest hurdle that most GMs cannot get past is that high level play is a completely different ball-game than low-level play. Stuff that worked at low-levels doesn't work anymore. Stuff that wasn't possible at low levels now is. What was once impossible is now do-able regularly. So as a GM you must prepare for the impossible. Sounds daunting, right? It's not that bad.
- Action Economy: Avoid combats that involve single or few opponents. Killing or incapacitating a single enemy at high levels is generally trivial. Make use of groups of enemies; minions, summons, waves of opponents arriving over multiple rounds. There are many reasons for this. From a role-playing and story standpoint, the player characters aren't normal anymore; they can each take on armies by themselves without breaking a sweat or taking a break - only groups of enemies pose any threat at all.
From a mechanical standpoint, it shifts the action economy into the bad-guys favor, at least for a while. Lots of enemies in D&D can pose threats to a group of characters without being a serious threat in the own right. A party versus a single enemy has an average of 4 actions to 1 action of the single enemy. Actions are what win high level play, and denying actions denies an opponent victory. Cannon fodder that grapples and harasses enemies is exceptionally useful for breaking action economy.
- Encounter Scenes: Epic heroes need epic scenes. Players should engage in battles where the environment around them is not only exciting, but dangerous in many cases. Trees of a forest are weapons opponents can use against them. Enemies can grapple players and take them for a swim in magma. Wyverns can grab a player and drop them in a spike pit full of insect swarms. Krakens can drag players into the depths. Enemies should rarely engage epic characters in simple slug-fests. Encounters inside rooms rapidly filling with water; or with pressing walls, or so forth can challenge players and also force players to use resources that would otherwise be used in combat (such as wall of stone //or //disintegrate). Three dimensional combat is also interesting. Players have to decide between fighting opponents above or below them, while also dealing with those on the same plane (excellent when in water, or dealing with incorporeal or flying creatures).
- The Monster Mash: Toss in different kinds of NPCs from time to time. Monsters are like player characters; they work better on teams; and high level foes know this. Enemies that can attack the players in multiple ways; and try to use multiple enemies whenever possible. A 16HD medusa with allies immune to her petrifying gaze, as well as some undead bruisers (like giant skeletons) to grapple and pester opponents can be a frightening encounter. While the skeletons likely aren't even a threat to the party, they can complicate things for the party - and that complication is a powerful inducer of fear and excitement.
- Casting Casualties: High-level casters are powerful and dangerous. Players know that, and intelligent enemies know that. In fact, almost everyone in a fantasy world should know that. Many of them are godlike in their own ways; and while they have a variety of ways to counter attacks and offensive tricks directed at them, each time they do they're using up resources (or more importantly, actions) which make it more and more difficult to keep up the pace. Enemies will prepare for spell-casters. A volley of arrows can render a mirror image spell void. Readied actions are their bane. A group of low-level mooks can afford a few charges of spells like lightning bolt or even CL 9 magic missile to activate when casters begin casting. Due to way Concentration works (that is, you must save against the damage you've taken during the casting), repeated hits require progressively larger DCs.
Vials of acid lobbed via slings are exceptionally good for this too. Same with alchemist fire, breath weapons, and so forth. Also, breaking line of sight or line of effect during the casting of something is often enough to drive a caster bonkers; since it often ruins their spell (or in the case of spells like fireball can end up detonating them too-close to the caster). Virtually anything that can nickle and dime a caster while they're casting will pester the hell out of them; and the more Concentration checks you can force the better. Also, melf's acid arrow applied liberally is an excellent method for stacking on not only immediate Concentration problems, but also lingering problems.
Example: A 20th level wizard is casting a 7th level doomsday spell. Immediately the 6 3rd level goblin adepts take their readied actions, each blasting him with a 10d6 lightning bolt from a wand (costing 450gp per charge from their NPC gear). The wizard is wared against 9 spell levels. The first three bolts eat away his spell protections, while the latter three deal an average of 35 points per bolt, which even if the wizard saves is about 17 damage per bolt. The wizard must make 3 Concentration checks - DC 27 / DC 44 / DC 61; since more and more damage was dealt during the action as the effects are applied. In addition to damage received.
Pathfinder Note: Concentration was removed in Pathfinder. It now is a caster level check + key ability modifier for the caster; meaning that most casters without feats cannot get more than a +35 to their checks at best (assuming 20th level and a +15 bonus from optimized key ability); so magic and masterwork items do not assist this.
- Be Devious: Don't play monsters like they're stupid (unless they are stupid). Have NPCs use their gear. Bad-guys should use lots of consumable items; NPC experts should Use Magic Device. Wands, bombs, oils of magic weapon and magic vestment, potions of enlarge person, mage armor, and shield. A single adept who casts bless on a group of enemies before a fight is practically like raising an entire group of creatures 1-2HD worth of levels as far as their attack bonuses are concerned. Haste is also brutally effective in this way. These are high level characters; they can handle it. Suicide bombers are particularly nasty. Hordes of goblins or 1HD skeletons charging the group while tattooed with explosive runes is particularly nasty; but can be stopped with clever shots with a bow. Also, covering an area with explosive runes makes waving around mass dispels a dangerous prospect.
- Use High HD Creatures: This one might sound weird, but it's incredibly useful for padding encounters and enemies. More than anything, a creature's saving throws, spell resistance, special attacks, and attack bonuses are based off their HD. Taking standard enemies and increasing their HD generally doesn't push them much higher than their usual challenge ratings. A 40 HD Aberration, construct, elemental, fey, giant, humanoid, ooze, plant, undead, or vermin often won't be any higher than CR 14, but can sport very solid base statistics before even applying ability modifiers and other considerations. Such opponents can force players to adjust their tactics to handle the high-stat creature; and attack it from lots of different angles. By angles, I mean things like negative levels, ability damage, speed reductions and so forth. They're exceptionally difficult to 1-shot; and often take a team to bring down. Especially if they have a modest SR (like 5 + HD), multi-targeting attacks, fast healing, and solid hit points.
Additionally, enemies like this inspire or urge players to use different tactics. Spell-casters would have difficulties with such foes (while not being useless against them by any means), while equipping the fighters with a life-drinker and a death-ward spell would allow him to lower the SR and Saving Throws of the beastie; while pummeling the creature's face. Enemies like this often require more than a single player or class to handle effectively.
- Let Players Be Cool: If the fighter in the group decides to use his adamantine sword to leap across a chasm and drive it into the wall for support; consider letting him do so and give him a bonus to his climb check. If he wants to hang himself on his sword by a belt while he fires arrows from the wall, let him try that (even with a -4 penalty to his climb check, but he's stationary so at that level it's not difficult). If a player wants to tear open a water-valve to wash away opponents, let 'em do it. If a player wants to cast fabricate to drop the bridge out from under an oncoming giant, let 'em do it. If a barbarian wants to tear down a tree or pillar with his weapon and then kick it over onto his opponents, let him do that.
Such actions aren't only not against the rules, they make use of them. Object Hardness and HP aren't that bad at high levels, and full-attacking and power-attacking is brutally effective and rending stuff apart; and using the terrain as weaponry is both a good option for high-level melee, and also a very cool one. Inciting cave-ins, collapses, and dropping lots of material on people is already covered in Cave-In and Collapses; and can be a clever way for martial characters to take enemies out of a fight using only their skills and physical attack power.
Be ready for my next article; part two in this short running series. In it, I will discuss handling high-level spell-casting, as well as discussing key obstacles to fun high-level play that you might wish to address. Hope to see you then.