How to protect the Cob Cannons

The key is that speed of zombie spawning is somewhat controlled by how fast you kill them, so try to make it as slow as possible. Potato mines are great early on. Snow peas are also good at doing slow damage. And of course you can just throw up walls for them to slowly eat through. Buy yourself enough time and you should have no problem getting enough sunflowers going.

Bobsled Bonanza/Strategies page at wikia lists a variety of strategies you can use.

Here are just a few of the strategies they suggest:

Alternate Strategy One: The player will be most vulnerable at the beginning. Get a double column of Sunflowers up fast and three in rows 1 and 6. Plant a Cattail after the first eight Sunflowers and as soon as they finish their cooldown after that. Plant Potato Mines as soon as they are available, but prioritize Spikerocks. If a Zombie Bobsled Team makes it past the ice in an unprotected row, drop a Wall-nut, wait for them to bunch up, and use a Squash. Once the lawn is under control, you can replace Sunflowers with more Threepeaters (on land) and Cattails (in the pool). Use the Squash, Jalapeno, Cherry Bomb, and Spikeweed As written here, this strategy includes 11 plants: 1 Sunflower 2 Lily Pad 3 Cattail 4 Potato Mine 5 Spikeweed 6 Spikerock 7 Wall-nut 8 Squash 9 Threepeater 10 Jalapeno 11 Cherry Bomb

Alternate Strategy Two: This strategy is so simple, nine words can describe it: two columns of Sunflowers and two columns of Gloom-shrooms. Nothing will get past it. [Note]The trick is building it. The iOS version only has 3 ice free columns, and the bobsledders will overrun you early. You need a lot of defense before you can build the columns.

Alternate Strategy Three: Plant eight Sunflowers on the first and second columns at the 1st, 2nd, 5th, and 6th rows. In front of the Sunflowers, plant at least two Peashooters at every lane. Plant two Peashooters for the pool lanes as well, on top of Lily Pad, to have Defense against Ducky Tube Zombies. Melt the ice as soon as possible with Jalapeno. After melting ice, add Spikeweeds and upgrade to Spikerocks as soon as possible to prevent more ice from coming in. Plant Cattail and Wall-nut or Tall-nuts for more Defense.

Alternate Strategy Four (the “Armageddon” method): Plants: Lily Pad, Sunflower, Wall-nut, Squash, Potato Mine, Jalapeno, Cherry Bomb, Doom-shroom and Coffee Bean. Plant a minimum of eight Sunflowers in the leftmost ranks.

Use the Squash (cheap) wherever possible to stomp Zombonis, and plant Potato Mines (cheap and quick to regenerate) as backup Zomboni killers and to take out entire Bobsled Teams. As things get more chaotic, move up to the heavier explosives. When things get really crowded, place a Doom-shroom on a Lily Pad midway down the pool, then drop a Coffee Bean on it to detonate and clear the yard nearly fence-to-fence. Clean up smaller groups with Cherry Bombs (if they are compact) or Jalapeños (if they are strung out along the file).

Bide your time and wait till the last possible moment to detonate your Doom-shrooms; they take a long time to regenerate and the longer you wait, the more zombies will enter the yard (and the kill zone). If you wait too long and miss a few, or want to wait longer to draw more in, or need a few more seconds for a plant to recharge or to gather more Sun, drop in a Wall-nut to slow ’em down, bunch ’em up, and give you more time to Squash ’em or blast ’em.

Try to reserve the Jalapeno as your ace-in-the-hole; if your explosives timing is off and you miss a few zombies, you can use it to zap the endangered file. If you can, wait while they eat a Sunflower or two (easily replaced) and you may also be able to nail a Zomboni or Bobsled entering from the right as a bonus.

Alternate Strategy Five: Pick the following plants: Sunflower Jalapeno Imitater Jalapeno Cherry Bomb Squash Repeater Lily Pad Spikeweed If you have 9 slots, pick the Torchwood, and if 10 slots, pick the Twin Sunflower. Procedure:

1.) Plant Sunflowers on the first two columns of the lawn (making you have 12 Sunflowers).

2.) When a Zomboni appears, plant a Spikeweed in front of it.

3.) When a Zombie Bobsled Team appears, use the Jalapeno to kill those four.

4.) If a Zombie Bobsled Team appears and your two Jalapenos are still recharging, plant a Repeater immediately. (I assume that you have enough sun for it at this point.)

5.) You must have planted Repeaters on the third column (and later on, probably on the fourth) before or during the next huge wave of zombies.

6.) Use the Squash or Cherry Bomb whenever some zombie get past your current defense.

7.) Only use Spikeweeds to kill Zombonis!

8.) Use again the Jalapeno when a row gets huge amount of ice. (Use the Imitater if the zombies are far away and use the real one if the zombies are near.)

9.) If you have enough sun and time, plant Torchwoods and Twiin Sunflowers.

If there are too many Zombie Bobsled teams in one row, use the Cherry Bomb in the middle of them. Reserve your Jalapenos for urgent uses. Don't waste them on rows with just ice and no zombies! If a Zomboni crushes your Repeater, use the Squash to prevent them from proceeding to your house.

Alternate Strategy Six: Plant Sunflowers in the back and build up Starfruit in the pool while you hold the initial waves off with Potato Mines, Squashes and Jalapenos. Once the Pool is full of Starfruit, nothing can get past them.

Also, here is a Youtube video that illustrates another strategy you could try.