The different types and mutations of the Zombies you use to infiltrate/invade in Zombie Farm can make a major difference in whether your zombies all die or you successfully infiltrate the enemy with minimal or no losses. Each enemy is slightly different, although in all cases it usually works best to have a variety of zombie types. However, you can be successful against the earlier bosses (especially Farmer) by just using the max amount of Mini Zombies/Zmurfs/ZomGoblins. As you get to tougher bosses (Pirates, etc.) it becomes more important to not only use a variety of Zombies for their skills, but also take advantage of mutations.
Old McDonnell’s Farm
The Farmer likes to throw stuff at your Zombies as they approach and attack the guys with pitchforks. The best strategy is to use fast Zombies, so you quickly have the Zombies back up your other Zombies and help beat the guys with pitchforks. One way to do this is to use the Mini Zombies/Zmurfs/ZomGoblins, as they move quickly naturally.
You can also use more powerful Zombies with speed mutations. For the best speed mutation combination plant zombies between carrot (+1 speed mutation) and coffee (+2 speed mutation) plants. Since the carrot is an eye mutation and the coffee is a head mutation a Zombie planted between both has a chance of getting both mutations.
You can also try to combine the head and eye mutations with arm, hair, and/or body mutations, if you plant the Zombie touching plants that give different types of mutations, although it is not really necessary for beating the Farmer.
Zombies vs. Lawyers (unlocks Level 16)
The lawyers are tougher than the Farmer, but the lawyer seems to be less aggressive in throwing stuff down at your Zombies. The Lawyers can also be beaten with just Mini Zombies, but you are likely to have a lot of losses and might not be successful. However, if you use Mini Zombies with mutations you will have a better chance of winning. Using more powerful Zombies that have longer grow times along with mutations can also make it easier to beat the Zombies.
Speed is pretty much always useful, but not entirely necessary with the lawyers. It still does help some to have a powerful Zombie go in followed by some fast Zmurf/ZomGoblin back up. The power and attack mutations are most useful against the Lawyers. The best attack mutation is the Dragon Arm Mutation, which gives +4 attack, but even the lower attack/power mutations can help give you an edge over the Lawyers.
The Tomato head (+1 power) or Garlic head mutation (+3 attack) can be combined with a Carrot Eye Mutation for some added speed along with a Turnip (+2 power) or Celery arm (+3 power) to give you a good mutation combination of attacking power and speed.
Zombies vs. Pirates (unlocks Level 21)
The Pirates are very strong and using Mini Zombies is pretty much useless even with mutations. The Pirates can kill the Mini Zombies in one hit. The best strategy is to use the more powerful Zombies, especially ZomBrute. The Amazombie can also be useful because they have more speed than the other big Zombies and can provide quicker back up than the slow guys. Send out a slow guy and then a few Amazombie followed by more of the slow, but powerful Zombies and beating the Pirates is possible.
Mutations become extremely useful to almost necessary when you take on the Pirates. The most useful mutations for taking on the Pirates are the ones that boost life, as the Pirates slowly attack, but each hit causes a lot of damage. By having more life your Zombies can last longer and be more effective at taking out the Zombies between hits.
The best life mutation is the Flytrap Mutation as it gives +4, since this is a neck mutation you can also combine it with the +3 life Lima Bean body mutation for maximum life, as well as the Cauli Hair Mutation for +3 more life. Also adding a power mutation, such as the Celery Arms mutation (+3 power) is also very useful. For a little bit of speed you could also try to a mutate a Zombie the life mutations along with a Coffee head (+2 speed) and Carrot Eye (+1 Speed).
Zombies vs. Ninjas (Unlocks Level 26)
The Ninjas are not quite as powerful as the Pirates, but they are tough and there are more of them. The life mutation can still be helpful, but since the Ninjas do damage slower than the Pirates, you might instead want to focus on speed and power/attack mutations.
The Lima Bean body mutation that gives +3 life is still good to use against the Ninjas, as it is the only body mutation. Try combining it with the Dragon arm mutation (+4 attack) or Celery arm mutation (+3 power) and Garlic head mutation (+3 attack) to maximize your attacking power to quickly beat the ninjas and get the boss to come down and stop putting up annoying carrots that you have to tap away. You could also use the Cauli hair mutation (+3 life) and Flytrap neck mutation (+4 life) if you feel you need Zombies with more stamina. A Carrot eye mutation (+1 speed) can be added for a small speed boost, too.
Using the Lima Bean Body mutation (+3 life) with the Dragon arm mutation (+4 attack), the Coffee Head (+2 speed), and Carrot Eye (+1 speed) is also a useful mutation combination against the Ninjas. This is especially good to use with the Amazombies, as quick back up to sending out a strong ZomBrute out first.
Zombies vs. Robots (Unlocks Level 31)
There are only two robots and the boss robot to defeat, but they sure are tough. It is not only necessary to use Zombies combined in the Zombie Pot, but also pretty important to combine mutations in order to successfully beat the Robots. Life and Power mutations are both useful in helping beat the robots. Make sure your Zombies are all hungry for brains or the delay of their distractions when not fully hungry can easily cause failure. Also, do not forget to keep tapping the head in background when he lights up to avoid him zapping your Zombies dead.
A great mutation combination to send out first is a Zombarian with a Flytrap neck (+4 life), Cauli Hair (+3 life), Garlic head (+3 attack), Lima Bean Body (+3 life), and Dragon Arm (+4 attack) mutations. This gives you a a strong Zombie that can stand the robot attack while you bring in reinforcements to help beat the robots.
Zombielocks are good to send right after the Zombarian, as they are quick reinforcements and with the right mutations they also have good attack and life. Use the same mutations of Flytrap neck (+4 life), Cauli hair (+3 life), Garlic head (+3 attack), Lima Bean Body (+3 life), and Dragon Arm (+4 attack) for Zombielocks. You might also want to add a little extra speed with a Carrot Eye (+1 speed).
Robo Zombies are good secondary reinforcements, as their laser eyes attack as they approach the enemies. They will do more damage and last longer with the Flytrap neck (+4 life), Cauli hair (+3 life), Garlic head (+3 attack), Lima Bean Body (+3 life), and Dragon Arm (+4 attack) mutations. Speed mutation is not really useful for them, as it will limit how much laser damage they do as they approach. Party Zombies with the same mutations as far as what they take (i.e. no head or hair mutations) are also quite useful.
Zombees are also very useful to use when battling the Robots, as they heal your Zombies as they battle and also once per battle can bring a Zombie back to life. Life mutations are the most useful for the Zombees to keep them alive healing your other Zombies as long as they can. Combine the Flytrap neck (+4 life), Cauli hair (+3 life), Lima Bean Body (+3 life), and Potatohead (+2 life) for the best Zombees.