"is millions of dead things buried in thick layers of waterborne sediment, and that is what we find"

But they're not mixed up. How did they sort themselves into clearly defined zones? What is the mechanism for this? What does "order of burial" actually mean?

"what is your evidence that it was not laid down within a year ?"

Sedimentary rocks come in a wide variety of types and these are laid down in a number of different ways. We do find flood events as well as lahars and other deposits laid down quickly, and we also find deposits laid down in tiny amounts that take aeons to accrue.

These layers can appear and disappear in a stratigraphic column as seas rise and fall, rivers meander and volcanic events occur. We find wonderfully delicate fossils that were laid down in very quiet environments with very fine sedimentation (Solhofen, Jehol) and we can find the poorly sorted deposits of alluvial fans and glacial moraines. The principles to look up are uniformitarianism (modern rather than Lyell's, revised to take neocatastrophic events into account) and superposition.

Flood events are violent, high-energy occurrences that deposit sediments in very distinct ways. Deposits tend to be poorly sorted and poorly stratified; smaller flooding events like levee breaks can deposit localised loads of suspended sediment (such as sand in a river) that settles as the energy decreases leaving sheet-like deposits. Rivers show very specific cycles of erosion and deposition, as do prograding deltas.

So we know some deposits take eons (abyssal plains, chalk etc), some take tens of thousands of years, some take days, some hours or minutes. All of this tells us there has never been (as far as we know from the evidence) a single worldwide flood event. Ever.

One more thing. The root of studying sedimentation is facies analysis, looking at the rocks to define the bed type and develop a hypothesis on how the scenario for the creation of a stratigraphic sequence. Apart from assuming the laws of physics and chemistry haven't changed there are no a priori assumptions allowed! Existing analysis must be tested robustly to ensure subjective interpretation is kept to a minimum. Question everything.

"Polystrate" is a creationist term and not one used by palaeontologists or geologists. As usual with creationist concepts this term seeks to apply one term to a wide variety of events, in this case the idea some fossils such as tree trunks span different beds. I could spend all day typing a reply, suffice to say the wikipedia page does a pretty good job of explaining these fossils and the numerous mechanisms that means this mode of preservation can occur.