I've been at it for weeks. My best is 26 waves without cob cannons.
Just finished first game with build that included two cob cannons on lily pads with nothing else except gatling pea shooters shot through fire and made it to 25 waves. Have to rethink rest of board as Gatling pea shooters and fire isn't enough. Think I must mix in two more cob cannons by wave twenty then go with cob cannons and two slots of doom shrooms the rest of the way out.
One good thing to always do though is plant two cattails to handle balloon zombies as cactus guns are useless for long term builds and blovers are a complete waste of precious sun points.
I despise the bungee zombies the most. Thieving turds have been known to attack in groups of six or more at one time. If the build is good though they can only get your sunflowers in the back row which are easily replenished.
Damn....I know entirely too much about zombies and plants. Time for a walk.