Beating Dr. Zomboss in the last level of Plants Vs. Zombies can certainly be challenging, especially with his powerful attacks. Here are some strategies to help you succeed:

1. **Plant Selection**: Choose plants that can withstand damage and have high attack power. Along with the Watermelon Pult, consider using:
- **Cabbage-pult**: It can deal good damage and is less costly.
- **Starfruit**: This can hit multiple lanes and provide good coverage.
- **Peashooters**: They can provide steady damage and are cheap to deploy.

2. **Defense**: Make sure to plant some defensive options in front of your high-value attackers:
- **Wall-nuts or Tall-nuts**: These can absorb a lot of damage from the zombies and buy time for your offensive plants to deal with them.
- **Spikeweeds**: Place these in the front lanes to deal with zombies that reach your defenses.

3. **Weather Control**: When Dr. Zomboss throws the snow/fireball, be prepared:
- **Tornadoes and Ice Shrooms** can help slow down the zombies, giving your plants more time to attack.
- **Plan your planting**: When you know he'll throw a snowball, try to have plants that can survive the blast in the back row, where they are less likely to be hit.

4. **Use the power-ups**: Make use of your sun production efficiently to ensure you can deploy plants quickly and keep the pressure on.

5. **Timing and Placement**: Place your plants in such a way that they're protected from his attacks. Rearranging the tactics you use based on the specific types of zombies he sends out can be effective too.

6. **Practice Patience**: Each time you play, you might learn something new about how to deploy your plants based on the specific patterns Dr. Zomboss follows.

With a good strategy and some patience, you'll be able to beat Dr. Zomboss and complete your adventure. Good luck!