Alright. I've come up with a Survival: Endless challenge. It requires a wide variety of skills and pretty precise timing. It's not too original, but it's pretty hard. Here it is. In fully separate games, without changing the "final" design (You can add Sunflowers and consumables, and repair if you need to), get to 100 flags in Survival: Endless with all the following types of builds:
Cobless w/ Firepea (requires at least one Gatling Pea on land and at least one Torchwood in the same lane)
Cobless, after "final" design is finished, unless something goes horribly wrong, don't bring Pumpkins ever
This 4 Cob build: (the Cob Cannons and the Twin Sunflowers can be rearranged)
12 Cob (at least 1 Cob Cannon must be vulnerable to Diggers)
Gloomless (no Gloom-shrooms)
Partial completions will be accepted.