Hi guys !
The only way to get 60 ships in one run is to use the 1UP pie.
Okay,here's the secret: On every level, you have a chance to go to the right path (the hard path).
On some levels you can find an MBG pie, and it will grant you score drops, as you shoot it (milk it).
On level 2 and 5,that path is special,it's "free flight" and there's a hidden MBG pie to find.
If you fly to the right the whole time,(so your ships are on the left side of the screen and aiming to the right.), then you will quickly find a hidden MBG pie and it drops... 1UPS!
If you shoot it, and milk it, and don't grab it too quickly, you can easily get 60 ships out of it. (possible in both levels).
I did it on Easy,alone with 2 controllers and with 150% speed.Very easy.It took me 5 minutes to get this achievement :)
Have fun !