General Tips:
-> Once you've got a first hold on the world and some money back to the Lab, you should always have a stocked Inventory. No need for perfect compositions, just some Bombs, some Goo and 3-5 Zombies of any kind, so if anything goes awefully wrong, you can compensate on a momens notice.
-> Don't be sparse with Consumables once you Encounter the First Z.I.T. Units. You get enough money by this point to use 2-3 Items per Map consistently and still make enough money for Research.
-> You can open and close doors by clicking on the Yellow Icon.
-> If you just need some more time to use Items, you can close all doors to an empty room and spawn a Zombie in it.
-> ALWAYS try to fully clear your Levels. You get a Bonus of 4 Random Items, ranging from some Gold to ANY kind of Zombie in up to a 3-pack you've unlocked by then.
-> BUT: Don't overexert your Ressources. Especially Early on, Money is precious, don't waste 5 Items and all your Zombies because you're starting infection failed completely. No Shame in calling it quits and preserving some Ressources for a better Level.
-> Never use Basic Zombies for Inventory, as it is needed for any and all upgrades. Take spare special zombies with you!
-> Infected Areas in the World will generate Basic Zombies for you. The more Areas you have already infected, and the higher their respective infection level, the more you generate.
-> Infecting everything a bit instead of pushing one specific City to Eradication gives you more time until the World get's harder, as Panic builds up much slower in lower Infection Rates than in Higher.
-> You don't need to instantly start a mission, you can also just sit on the World Map and Speed up the Game (Top right corner) to generate Zombies for you.
-> Try to push every Research facility to Eradication once you got to them, but don't be afraid to do another City first to get some money before making the final Push!
-> BUT: Beware! Research is still done whilst waiting on the Worldmap!
-> Every Continent has a Boss. Killing him will double the Infection Rate (The bar filling until you can eradicate a City), therefor halving the amount of levels you need to do to destroy a research facility.
-> Boss-Level gain all external Help, so better do them sooner rather than later, as they get exponentially harder!

Investment:
All Zombie Types have Base-Stats, which are altered by Basic Research. Some Zombies can't infect, others don't even attack peole, and Research won't change their Base-Stats or Behaviour!
-> Infection Chance is at all Stages KING. If you can't turn people, you are effectively conquering the World with 4 Zombies per Level.
-> At the Start you want to push Lifespan and Speed. A Zombie who can be outrun can't infect anybody, and If he dies after 5 Seconds he might as well not have been there.
-> Once the first Armament Rolls out, Defense gets important REALLY FAST. Attack should be pushed alongside, otherwise you can't kill anybody anymore at the same pace.
-> Mutation Chance gains Value based on your DNA Configuration. If you only got Crap Zombies unlocked, you might as well don't bother skilling it at all early on. If/Once youve got a nice Setup, Mutation Chance wins the Maps for you.

DNA-Upgrades will define the way you play the Game, so choose them carefully!
-> Always make the most of your DNA, you won't get much. Reshuffling ONLY in absolute WORST case!
-> Base-Stat Enhancements look juicy, but they gain far more Value over time once the modifiers get multiplied by high Research Stats, and are early on mostly outclassed by them, as Skilling Attack in the Research ONCE get's you multiple times more for the first half of the Game.
-> SINGLE EXCEPTION: Sight. Strongest Stat in the Game, always Skill as soon as you get the Chance!
-> Most Zombie-Type unlocks not only unlock the Zombie Type they describe, but also any Mix of it and all the ones you already have. Bear in mind what you already have, and want to push later on!

Research amplfiies your Zombie's Stats by a factor X, but if you got 0 Stats, the Result is still 0 even if the Factor is a million!
-> Always Focus on Leveling your Basic Zombie, as well as your Unique ones. They make up 85% of your Map and Strategy.
-> Don't forget to upgrade the Rest of the Zombies. If you mutate a Civilian into a Berserker, which in your Lab is Level 1, it will BE Level 1. If he is already at 5, we will whoop some Ass instead of keeling over.
-> NEVER Level the following Zombie Types: Coin-Collector, Zomblins, Clown. They Exist for their Specials, which don't scale with Level.

Unique Zombies:
Kruggy -> Spawn 100 Zombies
Jayson -> Kill 250 Humans
Mummy -> 100 Kills in the Desert (Middle-East Area, Rambeau Boss)
Yeti -> 100 Kills in the Snow (Russia and some close ones to it)
Goben -> Spend X Coins (no idea how much anymore, comes naturally)
Draculo -> XX Kills with Vampire Zombies (50 I believe)
Alien -> Unlock spitting, venomous and leaping Zombies (DNA)
Doomon -> Destroy a continent
Drumpf -> Kill Drumpf (USA Boss)

Setups:
Personal Preference:
->Grenade
->Mines
->Cannister (Secondary)
->Goo

->Stalker (3x)
->Yeti
->Kruggy
->Mummy (Early on: Jayson)
Creating a Pocket Infection in some remote Building or secluded Area, I protect the Horde from the Initial Contact Burst with Yeti, put Kruggy into them, and once colliding with the masses, spawn Mummy in the midst to overthrow and infect everything. The resulting horde mostly clears the Map itself, only needing help with Z.I.T. (Sometimes not) or any absolutely secluded people in other Buildings.
The Stalker is a personal Preference for worst case scenarios (As once you get the sight upgrade, he feels like a homing Infection on crack, finding everyone everywhere as long as there is a path to it), but If you are confident in your skills and got a maxed out Inventory, you might as well take Jayson instead for the Attack Damage Boost against Z.I.T. Units.