Is it just me, or is the OP saying "Bad builds suck ass at D&D"?
OBVIOUSLY, in a core only setting, it takes some party cooperation to defeat high level encounters. If, for some reason, you are arbitrarily deciding to play like a retard, then something like a storm giant is going to feast on your carcass and use your loot to fund a 6 month certificate course in "Profession: Cooking".
Outside of core, you can (rather easily, if you're an experienced player) make a character than can easily solo level appropriate CRs (barring some exceptions, like prepared wizards, etc).
It seems like you're arguing against the fundamental roles assumed by the game. None of the classes (Shut up, druid) are supposed to be able to go it alone. The action economy, if nothing else, provides a strong incentive for cooperation with a party. You can talk all day about how your uber casting build can handle any encounter, to which I retort: your DM must be taking it easy on you. Even druids can get wtfpwned by a clever DM, using only CR appropriate enemies.
Yes, I'm introducing DM fiat into the discussion. Well, it's the other half of the game, I think it bears mentioning.
No, his point is to try and figure out how melee-types can meaningfully contribute without having to rely entirely on the casters. As has been demonstrated, Team Monster tends to dominate in physical combat, to the point where unless you are literally one-shotting them they will own your ass without magical support.
As far as how easy it is to one-shot something, yes Brute monsters are fairly easy to one-shot outside of core, but spellcasting monsters are not. Especially not without magical support (BFC, buffs, etc).
And no, unless you are up against intelligently-played spellcasting monsters (this includes those with large arrays of SLAs), a well-played caster will run roughshod over them barring extremely unfavorable circumstances/luck. The DM has to intentionally stack the deck against you to make non-spellcasters a threat, and even that doesn't guarantee a victory for Team Monster.
DM fiat has nothing to do with it; casters, even in Core, straight-up wtfpwn non-casters.
Also, carnivore, your Core TWF Barb fails against the storm giant. I've spoilered the analysis below.
[spoiler]First, some numbers. Let's compare your HP: 130 (you, assuming average rolls) vs. 199 (giant); AC: 17 (you, assuming
haste and raging) vs. 27 (giant). AB: On a charge, you only miss him on a 1 (+24 plus +2 for charging), and he's only going to miss you one a 1 (AB +26). If you don't charge, you need a 3+, so that's a 15% miss rate vs. his 5% miss rate. Finally, let's look at your Ref save. You're sitting at +9 with
haste (this is relevant due to Awesome Blow). Now, let's look at the fight.
We'll assume you start 60' away from each other, and neither of you are surprised. You are
hasted and raging. The terrain is flat and basically featureless. You win initiative and charge him. He hasn't had a chance to ready an action, so all he can do is make an AoO against you. He deals 34.85 weighted damage. You meanwhile deal 21.88 weighted damage plus 1.25 weighted Con damage. On his action, he uses Awesome Blow and Power Attacks for 5. Even with the -9 to attack, his AB is still +17, meaning he deals 45.35 weighted damage (total 80.2 out of 130) and forces a DC 45 Ref save. You have a 95% chance of failing, whereupon he pushes you back 10 ft. and you are knocked prone. You are now 15 ft. from him and he still threatens you. On your action, you stand up and provoke an AoO, which he takes dealing a weighted 45.35 damage (125.55 out of 130). Either you move and provoke an AoO whereupon he kills you, or he finishes you on his turn.
If you succeed on your Ref save, he moves 35 ft. away from you, accepting the AoO, and the whole thing starts again from the beginning.
That's without him using any of his other abilities. Your one-trick pony fighter (seriously, you spent all of your cash trying to do this one thing, neglecting your defenses entirely) dies inside of 2 rounds against the storm giant and you barely scratched him.
If you had pounce you'd do much better, doing 106.14 weighted damage and 7.45 weighted Con damage on a charge, leaving him at 16.86 hp. At that point, a full attack (with no PA) from him would do 104.55 weighted damage, killing you.[/spoiler]