I had a hasted dwarf fighter annihilate my balor in one round with a full attack where he scored 3 critical hits and dealt out over 200 points of damage. (in case you were wondering, we make all of our "to hit" rolls on the table in front of everybody)
So, the fighter wipes an encounter because s/he rolls three 20s in a row, while the wizard does the same by casting a spell? Righty ho.
Try any single monster (non sludge) encounter where the monster gets hit by a Sword of Sharpness.
Or any single giant getting hit with a Giant Slayer sword
Or any Troll hit by a Sword of Wounding
Or any single dragon by a Sword of Dragon Slaying of the dragon's kind
Or any Titan by a Vorpal Sword to have him decapitated by a single stroke
Or any creature with mental capabilties by a Trident of Submission
Or any Paladin ignoring the effects of a spell due to his carrying of a Holy Avenger (50% Magic Resistance)
Or where a Fighter wields a Sun Blade against a horde of undead
Or a large group encounter where the Fighter uses a Net of Entrapment
Or where a Fighter wields a Mace of Disruption against a pack of wights
Or a large group underwater encounter where the Fighter uses a Net of Snaring.
Or a single opponent encounter where the Fighter rolled a 20 with his Life Stealing Sword or Sword of Nine Lives.
Or a single opponent who has just been hit by a Dagger of Venom thrown by the Fighter.
Or where a Ranger kills a single enemy with its particular Arrow of Slaying.
I could go on, there are many more weapons, many many more - and we havent even covered armour, warrior specific magical items...etc.
My point is NEVER say warriors NEVER really shined in combat, because then posts like these will happen. And its embarrasing.
What's embarrasing is that every single example requires a magic item, which the fighter's player has no say about the character getting or not getting. The wizard, on the other hand,
might be able to make it for the little tyke - bless 'im.
It is normal for sleep to blow through an encounter. It's not going to solve the adventure for you, but it will solve an encounter for you nicely. It can be fairly predictable, though it won't be constant.
Or an assassin's death attack: that'll kill a dude, if she pulls it off. She's an assassin, so she gets to pull that off on a fairly regular basis. It's predictable, though the circumstances under which it arises might require some coordination (disguises and stealth).
It's OK for an encounter to bite the dust, and even for an ability the character possesses to reliably and by design make a given encounter bite the dust...as long as the character can't do that ALL the time, to EVERY encounter.
Because you balance for the adventure rather the encounter, a character can't do that all the time -- you have abilities that won't come back after each encounter, or that can't be used in each encounter (though they can be typically used in each adventure).
Yes, I see where you are coming from, here, but the problem doesn't go away, it just becomes more subtle. Taking the three examples of fighter triple-critting a full attack, assassin making a death attack and wizard/MU casting sleep:
- the fighter gets to one-shot an encounter through luck, at a time chosen by the dice
- the assassin gets to one shot an encounter that consists of exactly one of a creature whose type is susceptible to their death attack, in other words, an encounter designed such that it precisely suits them
- the wizard gets to one-shot an encounter of his or her choice by casting a spell. At very low level this will likely be limited, due to the narrow spell selection s/he can load, but at by 5th level it ought to become "any one of the encounters the party is likely to meet" and by level 12 it will probably be "more than one encounter that the party is likely to meet".
Either the wizard/MU should be relying on luck/circumstance to enable their uber-power, or (better, IMO) the muggle-types should get much better control over when their "trump card" can be played.