Single character damage = 12.5
Group damage = 50-75
At-Will, not using any extra boosts.
That's at _1st level_.
The Tarrasque can't have less than 100 hp in that paradigm. It probably can't have less than 400, and frankly 1000 is probably more sane.
Yeah. So to reduce hitpoints, you'd need to reduce the damage dealt per hit, and/or the expected combat duration. I think both could do with a little reduction. And of course, you're not expected to hit every attack or even that you can actually bring all PC's fully to bear every round, so there's some mitigation there anyhow. Futher, before 4e an 18 was somewhat unusual and in 4e there's no 1.5 str modifier, so a +6 at first level is a little optimistic on average (though certainly achievable).
Some form of damage resistance or negation might also help provide resilience.
I think the really unnecessary bit is the high static modifier. It's just not necessary to roll 1d12+6 - either 1d12 or just 6 would play better (faster, and with rolls that either matter or are omitted). You could even go as extreme as RangerWickett's idea and use very low fixed damage, but that's probably a little too much for D&D.
Let's assume D&D want's to at least support damage dice up to 1d12 at first level, for histories sake. If we use damage dice only on crits, and a crit shouldn't do more than x4 damage at best, then we could get away with a static 4 damage for each attack. Assuming some form of damage mitigation and some misses, a 1st level DPR might be 2. Under these assumptions, a 1st level standard creature or PC might well have 10 hitpoints - focussed fire could bring it down in a round (as might a crit with a lucky damage roll), but on average you'd need 5 rounds.
At higher levels, creatures could have some damage resistance to reduce the need for inflated hit point counts whilst retaining the advantage of higher level creatures over lower level creatures. If an end-game (high-epic) PC deals 20 damage vs. a tarrasque with resist 10 damage, then a DPR of around 7 might be reasonable. With some regeneration, I'd say a combat duration of 5 rounds or might well be achievable even with just 100 hitpoints.
It's probably a little optimistic; there will be some extra inflation going on, but I'd rather have an aggressive target like a 100 hitpoints and miss a little than a target of 1000 hitpoints and end up with monsters edging towards 2000.