I'd be up for it AnotherLimey, send me a FR if you want.
Sod's Law...just after I asked how to get a Platinum medal, I got one last night
I managed it by playing Basic Training on General difficulty (you can't control the turrets, just place/repair them). I'd previously got all 3 Golds by not letting soldiers in my Toy Box but that alone wasn't enough for a Platinum. When getting the Platinum though, I used less turrets (ie, spent less money) and pressed 'X' to activate each wave as soon as it was possible.
I guess your profit and aggession are increased this way and needs to reach a certain level for a Platinum medal.