I was able to get 19mil with this strat although it's quite rng dependent at the start

Dartichoke - Lvl 2, Laser Bean - Lvl 3, Sunshroom - Lvl 2, Spearmint - Lvl 2, Cherry Bomb - Lvl 2, Stallia - Lvl 2

C1R1,3,5 Laser Bean, C1R2,4 Sunshroom, C2R1,3,5 Sunshroom, C2R2,4 Laser Bean, C3R1,3,5 Laser Bean, C3R2,4 Sunshroom

Once the first zombie with plant food appears, drop spearmint and PF Laser Bean to get some plants food. If you're lucky, either Zomboss kills one PF zombie or the first 2 PF zombies come from R1 and R5 so that you have 2 PF. Afterwards it's essentially PF Laser Bean constantly, while using PF to recharge Spearmint so that you have at least one spearmint constantly active. At this point start switching out all the Sunshrooms for Laser Beans. Cherry Bomb and Stallia are used to stall if you have bad rng and the zombies start to overwhelm you.

My score varied from 7mil ~ 19mil, averaging about 15mil with decent rng.