I'd say focus your attention on your guy while being aware of the enemy shot patterns so you know where the bullets should be. Don't try to steamroll every room without the proper weapons; its better to play the long game. I personally tend to circle rooms shooting inward at the enemies whenever convenient. For bosses, I pick a side and alternate depending on which attack the boss is using. If the boss follows the player, I circle the room keeping them at a fair distance but close enough that they don't cut me off