Fairly easy to get

Fairly easy to get.

Begin by adding sunflowers into the field to get extra solar power. I'd suggest having at least 1 row, maybe about 2 rows of them by the end.
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Once zombies start spawning use the rocknut as a shield to slow them down. They won't dmg the zombies but are cheaper and gives you time to build up more sunflowers or concentrate on a second lane.

Drop the spitters/freezers pets where needed, pay attention when ghouls spawn; drop strangler vines where you see and/or rocknuts to slow them down so you're ranged pets can slowly wear them down.