An easy way to overwhelm your opponent is to look at 1 Line of his cannons. Then spam opposite things of what his cannons can't hit. He will start to switch over his cannons to whatever virus you sent him first. Then spam a different virus. What I like to do is press QWERTYU at all diffrent times.
To all the noobs who says it's "OMG SO HARD HATE IT" etc. It's not difficult at all. Just spend all your money on Pumpgun ammo because each bird is like what.. 100 dollars each? And Pumpgun is 1 hit kill, just aim right. After that get about 300 bullets then you should be able to kil every single bird and until next round comes buy around 100-200 SMG bullets. I REPEAT do not use SMG always pump gun is faster than SMG.
When you first start buy the richochet gun. Best fastest gun ever with fast recharge. Kills Bosses/Buildings easy.
Also get the StepperStep or something. It's equivelent to the richochet except better.
Suggestion: Red Star [Russian] Platoon Commander, Rifle: 25-65, Rank: Commander, Weapon: TT-33 Tokarev Pistol, 25 TP, Special Ability: All troops gain 40% morale boost, 15% damage boost, All troops gain special taunts [I.E: Damn Krauts! (no offense anyone)]
Suggestion: Red Star [Russian] Shock Trooper, Rifle: 50-100, Rank: Sergeant, Weapon: Mosin-Nagant.
15 TP
Special Ability: Other Red Star Troops gain 10% Damage Boost, 5-7% Speed Boost, [5 or more Red Stars] 30% Morale Boost, [3 and more] All Red Star Troops gain "Potato Masher" Grenades.
Level 12: Make each square have equal amounts. Then After that place the last peice on top of the first 2[2 as in both square 1 and 2] blocks and it will stack onto your screens wall. GG
 "Mr Vengeance Act-I"){kind=icon}
 "Hacker vs Hacker"){kind=icon}
 "Damn Birds"){kind=icon}
 "Platform Racing 2"){kind=icon}
 "Heavy Weapons"){kind=icon}
 "Onslaught2"){kind=icon}
 "Minions"){kind=icon}
 "FlashTrek: Broken Mirror"){kind=icon}
 "Mud and Blood 2"){kind=icon}
 "Super Stacker"){kind=icon}