Medium-Extremely Hard (depends on skill level of the player or strategies)
Bobsled Bonanza is a difficult mini-game which can be unlocked after beating Adventure Mode. For some people, this is considered the hardest mini-game. It is a pool level which starts with ice on all four ground lanes (up to the fourth column), and almost all of the zombies that appear throughout the level are Zombie Bobsled Teams and Zombonis. If the player melts the ice, they will face several Zombonis before any Zombie Bobsled Teams. Potato Mines are good plants to use here, as a single one can take out a bobsled and all four of its occupants. Squash and Imitater Squash are also good plants because they can easily take out a Zomboni.
Bobsled Bonanza has a much longer time between the time the player is allowed to start planting and the time the zombies come than other levels. It lasts for 45 seconds.
The Tree of Wisdom says it's hard to beat both it and Pogo Party without the Squash. However, it is possible to beat the level without Squashes, because Potato Mines and Chompers can take their place. Also, Squash can only squash a whole bobsled if another zombie activated it at the right moment; by itself, it can only squash at most two bobsled zombies (and usually only one).
Unlike any other game modes, the ice trails don't disappear. In the DS version though, ice trails normally disappear after Zombie Bobsled Teams appear (unless a Zomboni is in the same lane).
For some reason, on some older iPhone versions of this mini-game, the ice at the start covers an extra square.
Gatling Peas also make a good use, but because of their high cost and slow recharge (plus that of the prerequisite Repeater), this isn't mainly a good idea.
A Repeater with a Torchwood cost 375 sun for the first one, and 200 more for each one. Also, all the plants will have a fast recharge. Gatling Peas with a Torchwood would cost 625 for the first one and 450 for every one after.