45 & 75 aren't that bad tbh... Just take all the towers quickly, then send a few waves to the central lab. When you have 150-250 and the boss has just sent a squad out, take the boss. Once you have that it's easy.
And the guy up top is a little wrong imo. Commando is the easiest to win with. Ammo belt + AUG HBAR is all you need to win lvl 14 and 15. Killstreak bouncing betty.
Two comments:
First off, the shield is the best 5k thing to get. It reflects all fire and projectiles and gives you magic every time you do it.
Secondly, the final boss is EXTREMELY easy. All you have to do is get the staff and spam the magic attack until he dies. Just have around 4 bandaids and ignore trying to dodge his attacks and you should win within a minute.
 "New Star Soccer"){kind=icon}
 "RPG Shooter: Starwish"){kind=icon}
 "JellyGo!"){kind=icon}
 "Epic Battle Fantasy 4"){kind=icon}
 "Strike Force Heroes"){kind=icon}
 "Relive Your Life"){kind=icon}
 "Billy the Pilot"){kind=icon}
 "Super Villainy"){kind=icon}
 "Flood Runner 3"){kind=icon}
 "Don't Look Back"){kind=icon}
 "Raze 2"){kind=icon}
 "Hero's Arms"){kind=icon}