Hi.
Ok, soilers and hints ahead
Regarding the door in 7.3, somewhere in stage 11 or 12, i am not sure, after the boss level you pick up a whole lot of trash, somewhere among the trash is a paper strip with a number on it, that's the door code.
Regarding the bombs, try walking on the platform above the playing field, the things won't hit you there.
The shield, use it by holding the down arrow, make sure that you are facing in the direction where the attack is coming from, else the shield will be of no use to you.
Also, watch for the endurance of the shield, you receive major damage when the shield breaks on you.
Greetings Moritz.
Hail the unholy church of Satan, go share it's greatness.