Not sure if this will fix it, but can't the following two lines be combined?
Code:
owerParatrooperTimer = [NSTimer scheduledTimerWithTimeInterval:.03 target:self selector:@selector(lowerParatroopers) userInfo:nil repeats:YES];
bulletTimer = [NSTimer scheduledTimerWithTimeInterval:0.03 target:self selector:@selector(checkCollisionAndMove) userInfo:nil repeats:YES];
And also for this line...
Code:
bulletCheckTimer = [NSTimer scheduledTimerWithTimeInterval:0.01 target:self selector:@selector(bulletCheck) userInfo:nil repeats:YES];
I recommend you replace the NSTimer with a CADisplayLink, youtube search it, there is a good tutorial on it. Otherwise everything looks good in the code you provided.