I call this path "the Stairwell" and it's what you'll want to build when you begin
Make sure there's an empty space between the bottom part of path and the screen to place your guns/towers

With your initial 600 breadcrumbs, you'll want to place 4 omni-blasters like so:

Finish the middle section of diagonal omni-blasters as you go through the waves of ducks
Place them when available

Upgrade that middle row of Omni-blasters, it may be hard to see but there will be a thin red ring around upgraded Omni-Blasters
Make sure all 6 are upgraded before continuing

Add a water balloon launcher (aoe gun) and honey-o-matic (honey gun) at the top by the toast
Then add a diagonal row of alternating:
aoe gun > honey gun > aoe gun > hot sauce splotcher (hot sauce gun) > repeat
on the lower portion of the Stairwell

Upgrade the previously placed row of alternating aoe, honey, and hot sauce guns from Step 5

You'll know if they're upgraded because the aoe guns will have a blue outline and the honey & hot sauce guns will have this sort of "/ \" on them

Same as Step 5 but on top side of Stairwell

Same as Step 6 but upgrade the guns placed in Step 7

Add two more alternating gun rows, as shown:

Upgrade the new rows from Step 9

Add a water gun at the top, left of the toast
And then a diagonal row of 5 water guns on both the lower and upper portion of the Stairwell

Upgrade the water guns from Step 11
They'll have a thin red outline when upgraded

Now add two rows of 4 snipers above and below the Stairwell
DO NOT UPGRADE THESE SNIPERS!!
The reason being that sniper upgrades extend distance from their initial radius to infinite range and we don't want that on these specific 8 snipers. Since snipers focus the highest health enemy, that means any upgraded snipers will only focus the entrance and we want these specific snipers to focus the highest health enemies on the path, itself.
So again DO NOT UPGRADE THESE EIGHT SNIPERS!!

Now we're going to fill the lower corner with snipers (be sure you upgrade these 6 snipers as soon as you place them; ie. if you place one and don't have enough breadcrumbs to upgrade it? Sell it, then wait til the next wave to re-place and upgrade it)

Note: it may take a few waves to fully place & upgrade all 6 snipers

Same as Step 14 but for the top right corner

That's it! Congrats!
Feel free to let me know if this guide worked for you and what changes you may have made that improved it :)
Good luck, brave toast defenders!