What i found to be fastest for this was to play in (timed mode)
keep getting combos around the bottom so when new tiles come in you get the free combos they give you sometimes.
Also try to go for starflowers as they are very valuable.
Save them for wave 7 or later as that is when you will get most points for them.
My best score was 235,000 in 25 minutes.
On average i will earn 100,000 every 15-20 minutes.
If you find anything wrong with this guide please i.m. Me and i will improve or fix whatever the problem may be.
Good luck and go get that achievement :)