i aimed to do it with reds as there seems to be more of them. just remove as many blue and yellows etc as possible to get a solid red brick wall. also keep pressing the little button at the bottom of the screen (just bellow where it shows you what coloured blocks are coming next), this button will bring the next line onto the screen.
8 blocks fit on each line so you need 3.75 full lines of red (or your chosen colour) before removing them.
Now this is the pain the ass bit, keep bringing more blocks onto the screen removing the blocks that aren't red untill you have between 30 and 39 blocks inclusive, then tap the red blocks!
Thanks to some very quick feedback from MICKERR
we now know these achievements
DO NOT STACK!! they must be achieved one at a time so keep a count of your bricks!
for this achievement you must fall within 30-39 blocks for it to unlock
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