Oh, gee, I knew this would come up eventually.
First off, a 45 degree swing is way off, it should be less than 30 (estimate), so I think your coach has that wrong.
Second, a completely level swing is never good, it means you dip your bat during your swing, making it longer and robbing you of your power and sometimes even accuracy with your swing. Your bat should travel on a straight line, from your load-up point to the contact with the ball.
Third, Greg, an uppercut is even worse, dropping your hands that low will make your batting average drop faster than a lead balloon.
Now, there's a difference between chopping the ball with a downward swing (which is the ONLY GOOD swing) and making solid contact with one. When you chop the ball into a grounder, you are simply making contact with the ball on top of it (like when you pop out when you hit under the ball). Even with a downward swing, you have to hit the ball in it's center to fully transfer the power and not lose any to spin, or... the ground. If you aren't hitting liners off the tee, then you aren't hitting it right.