What’s bacon good for

This website lists a bunch of strategies. Here is one example of a strategy:

Essentially, at higher levels, almost all strategies rely on having a constant supply of Cob Cannons to take out the Gargantuars with. The Winter Melons are used to slow down incoming enemies. The Gloom Shrooms in the pool are to damage pool-dwelling zombies and also attack zombies in the second and fifth rows. The Gloom Shrooms near the house are to kill Miner Zombies in the event that they spawn, and the Banana Leaves are to protect against Catapult and Bungie Zombies. The Ice-Shroom is placed in the pond to wake up with a Coffee Bean in the event that it needs to be used, Spikerock is to protect against vehicles and zombies that get too close to the Cob Cannons, and the Twin Sunflower to generate sun.

Here are two strategy guides intended to be used for Classic/Extended gameplay on hard but which are applicable to Marathon Mode:

http://iphonestrategygames.pbworks.com/Fieldrunners+-+Grassland+-+Extended+-+Hard http://www.iphonestrategygames.com/strategies/fieldrunners-grasslands

Good luck!