Old McDonnell is pretty easy below level 4, since his throw speed is reduced (1 item / 2 sec). Because of this you easily can beat him with every possible combination of zombies. You don't even need mutations. At these levels, it's recommended to invade often and build up a strong, mutated army. When you reach level 10, he will start throwing faster, and it becomes harder to win with Green Zombies. When you have enough Brains, you can buy a Blue Grave and grow Blue Zombies, and then Old McDonnell is easy again.
From level 10 Old McDonnell throws things at a very fast rate (1 item / 1 sec), and at level 15 his speed will increase again. It'll be 2 items/1sec .So it might be wise to choose faster moving zombies and/or using speed mutations so they don't get hit as often on their way towards the enemy. It is not recommended to use Garden Zombies before the boss descends. His projectiles will kill them easily since he always targets them first.
At level 21 Old McDonnell can be easily beaten with a mix of red and blue zombies as this Wiki's suggestion of "How to beat the Pirates?" Stun Stratergy. With most having 3-4 mutations zero zombie losses should be easy to achieve. However with only green zombies at that level is difficult and no guides are successful with them.
If an invasion fails, Old McDonnell will say:
- e-i e-i owned
- Zombies be gone!
- Brush yo' teeth!
- For the glory of love
- You lost?