Yep, you are right, and it´s that the reason to limit the shield only to 9 seconds in all uses. Using wisely, 9 seconds are a lot ;). Of course, I played a loooooot of games, and if you want to survive long enough you need how to use the box in combo with shield and jump to smash enemies as quick as you can (it has more uses than you think hehe). Without using a "hack", I think that it´s impossible to survive the total 99 seconds...
Thank you for playing my game and for your valious feedback ^^