There is talk about this subject occasionally, so the other day it dawned on me that I should ask our old DM, the guy who ran our high-level games how to deal with such things. This is what he had to say (tidied up for posting):
Best ways to work it are to spend the first couple of sessions with working the backstory of the campaign and getting to know the characters that your players have developed. Give them some information-gathering tasks with a couple of minor encounters thrown in to see what they are capable of...What are their strengths, weaknesses, and tendencies in combat. Then tailor each week to challenge the characters in their weaknesses while rewarding their strengths. It forces parties to develop teamwork and group-thought processes to overcome what is in front of them. Not every battle needs to have that invincible foe, but if it has one minor foil that can expose a character flaw (whether it be a combat flaw or a fear/phobia), you can really provide a challenge in how to overcome that distraction. Some trial and error is inevitable. You may find that a particular battle becomes dominated by [the DM's] forces or [the party's forces]. If it's [the DM's] forces, then you can keep morale up by providing means of escape before the whole party dies. Or if they escape on their own, some hidden information on how to turn the tide in their favor at a future encounter. Hope this helps!I hope this helps, too. It's quite a mature way of thinking for some young adults back in the 90s.