The Sparta achievement is very easy.. Play on normal (doh), buy 3 missile turrets, buy 60 gunmen and start the game. You don't even have to fire a gun. The gunmen and turrets do all the work. Easiest 15 points ever.
The campaign was way harder than killing 300 zombies.. Just keep spinning circles.. Also, avoid the big blue guys, cause if you hit those, you'll probably die. Anybody can get that achievement in less than 5 minutes.
The red bridge is easy, just do this:
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Tile 1 being most left and tile 6 being most right. If you want to go back, just do the exact opposite order.
 "Arcuz - Behind the Dark"){kind=icon}
 "Storm the House 3"){kind=icon}
 "Triangles"){kind=icon}
 "The Tickler"){kind=icon}
 "Hedgehog Launch 2"){kind=icon}
 "Gateway II"){kind=icon}