In most MMOs that's mostly true. Its less true in real time fighting games where the duration of the fight is in and of itself a skill challenge. For those familiar with MCOC, I currently have no problems taking down RoL Winter Soldier with X23. But Captain Marvel? She's a pushover in normal combat but when she has half a million health? All it takes is one mistimed evade on her L2 flippy attack and I be dead. I can dodge that attack 95% of the time. But to finish that fight, I would need to improve to about 98%.
That doesn't mean its a particularly *good* way to increase difficulty. Just that it does increase real difficulty.
If I was to rate a fight in which you had to do somethin perfect for 10+ minutes I'd say it's absolutely bad difficulty
.
A general rule of difficulty by design, is that a fight itself is designed to last a specific amount of time. Developers who make a fight very hard but last, say, two minutes have to take care not to increase that length when adjusting it's difficulty. The general rule is to truely obey "Difficulty by design" one has to instead change the AI and general behaviors of a fight rather than it's length. For example, starcraft 2 on brutal mode vs Warcraft 3 on hard mode. Starcraft 2 on brutal mode is by every definition a difficult game by design, instead of seeing enemy units given more health, they instead are supported by a wider variety of technologies and other units not seen on lower difficulties.
As an example of this in the game, I'll use 2 levels: Zero Hour, and Welcome to the Jungle.
In Zero Hour, on normal you only face zerglings, some hydralisks and mutalisks. On Brutal? BANELINGS show up to wreck everything and roaches are included to try and draw fire from more vulnerable units. On the defense, the zerg get ultralisks(yes, ultralisks) and of course you also face more spine crawlers. The AI also gets another deadly trick: It spreads creep on the map, increasing the speed of it's own units. So winning on brutal is a test ultimately of production and management of defense; you have to be able to construct your armies at peak efficiency.
Now lets go to Welcome to the Jungle.
In WttJ, the protoss on normal mostly use scouts, some void rays, zealots and stalkers. Nothing to severe, the player can make decent use of the otherwise not so useful goliath. But on brutal, a strong infantry becomes more important and any extra technologies are also important; the protoss gain access to collossus and high templar, both units which can easily destroy the necessary infantry(as void rays are that big of a problem for goliaths). Now, normally, given the goliath is meant to be an anti-air and isn't so effective, i'd say this mission sucks. But, smart use of your units and positioning as you approach a group of protoss can still make a large difference; both collosus and high templar are not very effective against marauders, and marauders are extremely strong against the collosus and stalker, those two units dying quickly enable the marines to rapidly fire down the void rays without to much trouble.