This level is not very hard. Celery Stalker, although it does a huge amount of damage, but it can easily be overwhelmed by hordes of zombies or zombies with special ability such as the MC Zom-B, so it should be treated as the early offensive plant. Cactus however can attack at better distance, so it should be the main offensive plant. The player also may think about feeding the Cacti Plant Food to make the level easier.
Due to Thyme Warp's very slow recharge, the player should only use it when things get too tough.
When zombies start to come out, plant Celery Stalkers on the sixth column from the left. This gives you a bit of time to build up sun production, but it lets you keep the zombies at a safe distance from your Sun-shrooms, and it allows you to plant Cacti fairly freely behind them.
Once a group of zombies comes out that a Celery Stalker normally cannot handle, plant a Cactus. Continue planting Cacti in the third, fourth, and fifth columns.
If the going gets though, use a Thyme Warp to send every zombie back. Once you strategy is complete, it should look like the following: