In the first turn the Hunter went first, she made 3 basic attacks each hitting with a roll of over 40 to shoot the three chains holding up a large statue hanging from the roof above the dragon. It hit for tonnes, but I secretly hand-waved that damage and gave the dragon no damage. The Scout charged in and hit the prone dragon for 86 damage, then the Slayer closed and hit it for 165 in MBA->Rain of Blowx3->MBA flurry of attacks with headsmans chop feat. Then the leader gets them to attack again, it was horrible.
This doesn't really sound horrible to me - folks worked well together, did solid but not crazy damage. Never be afraid to let PCs do solid but not crazy damage. It just makes the game go faster. Assuming that the leader didn't give them enough to kill it, anyways, but that sounds like only enough damage to bloody it, then they'd take about 2 more rounds to finish it unless their initiatives line up perfectly against it. Which, frankly, it does sound like, if it's acting 3-4 times per round and never gets to stand between their actions to avoid the headman chop damage
Granted, it might have missed _a lot_ if it made its 6 attacks (4 heads plus 2 APs), each of which targets 2-3 characters, and did only 45 damage (so 2 hits).
I have also noticed that there are a lot of abilities that are balanced in a group of 4-5 but are actually way more powerful solo. For instance a Cunning Sneak rogue could RAW take out most solo creatures in the game without a hassle. It is near impossible to stop them moving and hiding after every attack. They have a level 5 daily power that slows (save ends) on every single attack they do for the rest of the encounter. Just hit an enemy with that from 50 squares away and watch them die helplessly unable to catch or hurt you. The DM has to craft encounters where they can never bring their full power to bear.
I can't imagine that trick working all that well, especially not at the levels you're talking about. Regardless of "crafting encounters", stealth just isn't that threatening and only improperly designed monsters have no ability to cope with ranged attacks. For clarity, I've DMed for a permastealth character from L7 - L30; one who never even unhides, unlike a normal cunning sneak rogue who does appear when they attack.
I mean, you could have a hunter slowing the monster, but you could also just have an elf with a bow running at full speed. If it can't cope with that, it doesn't deserve to have made it past 3rd level.
Snipers are great, no doubt, but a sniper without support in hostile territory shouldn't have an option to just sit 50 squares away whittling someone down for minutes. I mean, even a stupid monster would go in a building or cave, and any allies at all mean you can cordon off an area or outflank. It's actually great from a plot standpoint (yay scouting, sniping lookouts, etc) but from "I went 1 on 1 with a solo and it could do nothing" that sounds like crazy tofu logic. Even if the Tarrasque has to pick up a boulder and throw it, it has options.
As the OP found, another example is the Bladesinger. Their ability to repoiste and have high defenses is balanced in a fight where the enemy can choose to hit their friend. Get them alone and the enemy is left with no good options.
Ready an action to attack on their turn. That's also an option that works on the rogue, often enough.
That said - yeah, D&D is a team game. If you break that mold you might hit some odd things.
