Vanquishing Gargantuars, along with the 3 Z-Mechs, as far right as possible will provide the higher scores...
I use Explode O'Nut to stop Zombies from triggering the lawnmowers and with Gargantuars and the 3 Z-Mechs when available (along with the Primal Potato Mines). Escape Root is also a good choice for Boosterama since you receive an extra instant explosion ! Bombard-mint early when ready but save Plant Food for the third Z-Mech.
August 2019 - last edited August 2019
Sap Fling Boosterama Mastery plants 2.7M best score out of over a dozen attempts so far
(average score over 2.2M)
(one little adjustment that added >300k to the score was placing Chard Guards in the far right column (C9) as the match proceeds)
initial layout is Chard Guards in column 8...once Let's Rock is 'on' then place Laser Beans where there are Zombies arriving on the lawn and spam Blover...just remember the same 20 second spacing of Imp Bots starting about 2:35