That closing wall trap is still getting me!
Also, the bubbles are cool. Instead of killing you, they block your shots. I like to let them absorb me as soon as I see them and get rid of them. They introduce another unique sense of interplay!
There's also everyone's favorite obstacle in this stage, edible walls. They feel relevant with the bubbles, though. Note that they're always placed in tight corridors, where you need to get through as quickly as possible before an enemy fills that space with bullets. It's not Instead, they're well-placed delays that emphasize the importance of making it through those tight spaces.
I just made it to the boss on the last life of my last credit today. I can tell it'll be a while before I clear this guy!